Don't read past end of string in Guess ctor

<https://ci.libreoffice.org//job/lo_ubsan/1082/>:
> ==26422==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x604000accc72 at pc 0x2ae43e63f4b6 bp 0x2ae43e600510 sp 0x2ae43e600508
> READ of size 1 at 0x604000accc72 thread T70 (cppu_threadpool)
>     #0 0x2ae43e63f4b5 in Guess::Guess(char const*) /lingucomponent/source/languageguessing/guess.cxx:95:28
>     #1 0x2ae43e667f2f in SimpleGuesser::GetManagedLanguages(char) /lingucomponent/source/languageguessing/simpleguesser.cxx:169:19
>     #2 0x2ae43e668420 in SimpleGuesser::GetAvailableLanguages() /lingucomponent/source/languageguessing/simpleguesser.cxx:179:12
>     #3 0x2ae43e64a18e in LangGuess_Impl::getEnabledLanguages() /lingucomponent/source/languageguessing/guesslang.cxx:229:24
[...]
> 0x604000accc72 is located 0 bytes to the right of 34-byte region [0x604000accc50,0x604000accc72)
> allocated by thread T70 (cppu_threadpool) here:
[...]
>     #7 0x2ae43e65350a in std::string::operator+=(char const*) /home/tdf/lode/opt_private/lib/gcc/x86_64-unknown-linux-gnu/5.2.0/../../../../include/c++/5.2.0/bits/basic_string.h:3355:16
>     #8 0x2ae43e667e6e in SimpleGuesser::GetManagedLanguages(char) /lingucomponent/source/languageguessing/simpleguesser.cxx:168:21
>     #9 0x2ae43e668420 in SimpleGuesser::GetAvailableLanguages() /lingucomponent/source/languageguessing/simpleguesser.cxx:179:12
>     #10 0x2ae43e64a18e in LangGuess_Impl::getEnabledLanguages() /lingucomponent/source/languageguessing/guesslang.cxx:229:24
[...]

shows, during UITest_librelogo, the Guess ctor making wrong assumptions about
the structure of guess_str and skipping over the terminating NUL.  Locally I
could see that while most inputs do have the expected syntax of starting with
"[" and containing two "-", one input is indeed just "[haw-utf8" without a
second "-".

I don't know where the strings passed into the Guess ctor in the two places in
lingucomponent/source/languageguessing/simpleguesser.cxx ultimately come from,
and what their guaranteed syntax and their semantics is.  So from the existing
code and the non--well-formed "[haw-utf8" sample (where the second segment shall
apparently designate an encoding, not a country), construct rules how to
robustly parse any input into potential language/country/encoding parts.  (What
is obvious from the call sites is that for one each input will start with "[",
and for another the item to parse need neither be "]"- nor NUL-terminated.)

(Guess::encoding_str and the local enc variable have effectively been unused
ever since the code's introduction in 07628119
"INTEGRATION: CWS languageguessing".  Guess::encoding_str, but not the local
enc variable, got later removed with b275246c
"loplugin:unusedfields in formula..registry".)

Change-Id: Icbedc05ed5b119ee4efbc3118cc17076a4d80c74
Reviewed-on: https://gerrit.libreoffice.org/62390
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

lingucomponent/source/languageguessing/guess.cxx

@@ -17,6 +17,9 @@
  *   the License at http://www.apache.org/licenses/LICENSE-2.0 .
  */
 
+#include <sal/config.h>
+
+#include <cassert>
 #include <iostream>
 #include <string.h>
 
@@ -39,11 +42,6 @@
 
 using namespace std;
 
-static bool isSeparator(const char c){
-    return c == GUESS_SEPARATOR_OPEN || c == GUESS_SEPARATOR_SEP || c == GUESS_SEPARATOR_CLOSE || c == '\0';
-}
-
-
 Guess::Guess()
     : language_str(DEFAULT_LANGUAGE)
     , country_str(DEFAULT_COUNTRY)
@@ -59,48 +57,47 @@ Guess::Guess(const char * guess_str)
     : language_str(DEFAULT_LANGUAGE)
     , country_str(DEFAULT_COUNTRY)
 {
-    string lang;
-    string country;
-    string enc;
-
     //if the guess is not like "UNKNOWN" or "SHORT", go into the brackets
     if(strcmp(guess_str + 1, TEXTCAT_RESULT_UNKNOWN_STR) != 0
        &&
        strcmp(guess_str + 1, TEXTCAT_RESULT_SHORT_STR) != 0)
     {
-
-        int current_pointer = 0;
-
-        //this is to go to the first char of the guess string ( the '[' of "[en-US-utf8]" )
-        while(!isSeparator(guess_str[current_pointer])){
-            current_pointer++;
+        // From how this ctor is called from SimpleGuesser::GuessLanguage and
+        // SimpleGuesser::GetManagedLanguages in
+        // lingucomponent/source/languageguessing/simpleguesser.cxx, guess_str must start with "[":
+        assert(guess_str[0] == GUESS_SEPARATOR_OPEN);
+        auto const start = guess_str + 1;
+        // Only look at the prefix of guess_str, delimited by the next "]" or "[" or end-of-string;
+        // split it into at most three segments separated by "-" (where excess occurrences of "-"
+        // would become part of the third segment), like "en-US-utf8"; the first segment denotes the
+        // language; if there are three segments, the second denotes the country and the third the
+        // encoding; otherwise, the second segment, if any (e.g., in "haw-utf8"), denotes the
+        // encoding:
+        char const * dash1 = nullptr;
+        char const * dash2 = nullptr;
+        auto p = start;
+        for (;; ++p) {
+            auto const c = *p;
+            if (c == '\0' || c == GUESS_SEPARATOR_OPEN || c == GUESS_SEPARATOR_CLOSE) {
+                break;
+            }
+            if (c == GUESS_SEPARATOR_SEP) {
+                if (dash1 == nullptr) {
+                    dash1 = p;
+                } else {
+                    dash2 = p;
+                    // The encoding is ignored, so we can stop as soon as we found the second "-":
+                    break;
+                }
+            }
         }
-        current_pointer++;
-
-        //this is to pick up the language ( the "en" from "[en-US-utf8]" )
-        while(!isSeparator(guess_str[current_pointer])){
-            lang+=guess_str[current_pointer];
-            current_pointer++;
-        }
-        current_pointer++;
-
-        //this is to pick up the country ( the "US" from "[en-US-utf8]" )
-        while(!isSeparator(guess_str[current_pointer])){
-            country+=guess_str[current_pointer];
-            current_pointer++;
+        auto const langLen = (dash1 == nullptr ? p : dash1) - start;
+        if (langLen != 0) { // if not we use the default value
+            language_str.assign(start, langLen);
         }
-        current_pointer++;
-
-        //this is to pick up the encoding ( the "utf8" from "[en-US-utf8]" )
-        while(!isSeparator(guess_str[current_pointer])){
-            enc+=guess_str[current_pointer];
-            current_pointer++;
-        }
-
-        if(!lang.empty()){//if not we use the default value
-            language_str=lang;
+        if (dash2 != nullptr) {
+            country_str.assign(dash1 + 1, dash2 - (dash1 + 1));
         }
-        country_str=country;
     }
 }