warn on \0 embedded in string literals, after all

Seeing 791f2768 I've changed my mind,
some embedded \0 can be actually well hidden:
struct foo { const char txt[3]; };
const foos[] = { { "a" }, { "bb" }};
If somebody wants an embedded \0 in a string literal, they need to
say it explicitly by specifying the size.

sal/inc/rtl/string.hxx

@@ -196,7 +196,9 @@ public:
       New string from a string literal.
 
       If there are any embedded \0's in the string literal, the result is undefined.
-      Use the overload that explicitly accepts length.
+      Use the overload that explicitly accepts length or cast the literal
+      explicitly to const char*.
+
       @since LibreOffice 3.6
 
       @param    literal       a string literal

sal/inc/rtl/ustring.hxx

@@ -190,6 +190,9 @@ public:
       is not pure ASCII, it needs to be converted to OUString by explicitly
       providing the encoding to use for the conversion.
 
+      If there are any embedded \0's in the string literal, the result is undefined.
+      Use the overload that explicitly accepts length or fromAscii().
+
       @param    literal         the 8-bit ASCII string literal
 
       @since LibreOffice 3.6

sal/rtl/source/strtmpl.cxx

@@ -1220,6 +1220,8 @@ void SAL_CALL IMPL_RTL_STRINGNAME( newFromLiteral)( IMPL_RTL_STRINGDATA** ppThis
             /* Check ASCII range */
             SAL_WARN_IF( ((unsigned char)*pCharStr) > 127, "rtl.string",
                         "rtl_uString_newFromLiteral - Found char > 127" );
+            SAL_WARN_IF( ((unsigned char)*pCharStr) == '\0', "rtl.string",
+                        "rtl_uString_newFromLiteral - Found embedded \\0 character" );
 
             *pBuffer = *pCharStr;
             pBuffer++;