Do not forget to actually set newStr to an empty string

(cherry picked from commit f7a7a9dd)
Conflicts:
	sal/qa/rtl/strings/test_strings_replace.cxx
	sal/rtl/ustring.cxx

Change-Id: I745b09d8a248f08afdd3387f4cfcf69d71ec3c39

sal/qa/rtl/strings/test_strings_replace.cxx

@@ -186,6 +186,9 @@ void Test::ustringReplaceFirstAsciiL() {
                  rtl::OUString("other"), &n)));
         CPPUNIT_ASSERT_EQUAL(sal_Int32(-1), n);
     }
+
+    CPPUNIT_ASSERT_EQUAL(
+        rtl::OUString(), rtl::OUString("xa").replaceFirst("xa", OUString()));
 }
 
 void Test::ustringReplaceFirstAsciiLAsciiL() {
@@ -225,6 +228,9 @@ void Test::ustringReplaceFirstAsciiLAsciiL() {
              replaceFirst("bar", "other", &n)));
         CPPUNIT_ASSERT_EQUAL(sal_Int32(-1), n);
     }
+
+    CPPUNIT_ASSERT_EQUAL(
+        rtl::OUString(), rtl::OUString("xa").replaceFirst("xa", ""));
 }
 
 void Test::ustringReplaceAll() {
@@ -265,6 +271,9 @@ void Test::ustringReplaceAllAsciiL() {
         rtl::OUString("xaa").replaceAll(
             "xa",
             rtl::OUString("xx")));
+
+    CPPUNIT_ASSERT_EQUAL(
+        rtl::OUString(), rtl::OUString("xa").replaceAll("xa", OUString()));
 }
 
 void Test::ustringReplaceAllAsciiLAsciiL() {
@@ -282,6 +291,9 @@ void Test::ustringReplaceAllAsciiLAsciiL() {
         rtl::OUString("xxa"),
         (rtl::OUString("xaa").
          replaceAll("xa", "xx")));
+
+    CPPUNIT_ASSERT_EQUAL(
+        rtl::OUString(), rtl::OUString("xa").replaceAll("xa", ""));
 }
 
 }

sal/rtl/ustring.cxx

@@ -1154,8 +1154,8 @@ void rtl_uString_newReplaceFirstAsciiL(
         }
         sal_Int32 n = str->length - fromLength + to->length;
         rtl_uString_acquire(str); // in case *newStr == str
+        rtl_uString_new_WithLength(newStr, n);
         if (n != 0) {
-            rtl_uString_new_WithLength(newStr, n);
             (*newStr)->length = n;
             assert(i >= 0 && i < str->length);
             memcpy(
@@ -1197,8 +1197,8 @@ void rtl_uString_newReplaceFirstAsciiLAsciiL(
         }
         sal_Int32 n = str->length - fromLength + toLength;
         rtl_uString_acquire(str); // in case *newStr == str
+        rtl_uString_new_WithLength(newStr, n);
         if (n != 0) {
-            rtl_uString_new_WithLength(newStr, n);
             (*newStr)->length = n;
             assert(i >= 0 && i < str->length);
             memcpy(