Make it compile with -Werror on gcc 4.6

public/common/unicode/unistr.h

Index: public/common/unicode/unistr.h
diff --git a/public/common/unicode/unistr.h b/public/common/unicode/unistr.h
index 57ea34652d67cfba6eba22b1b584bdbfe07ad197..9aba6f09a4087ab8809d68aaf533a29939149f9a 100644
--- a/public/common/unicode/unistr.h
+++ b/public/common/unicode/unistr.h
@@ -4023,6 +4023,7 @@ UnicodeString::replace(int32_t start,
   int32_t count = 0;
   UBool isError = FALSE;
   U16_APPEND(buffer, count, U16_MAX_LENGTH, srcChar, isError);
+  (void) isError;
   return doReplace(start, _length, buffer, 0, count);
 }
 
@@ -4326,6 +4327,7 @@ UnicodeString::append(UChar32 srcChar) {
   int32_t _length = 0;
   UBool isError = FALSE;
   U16_APPEND(buffer, _length, U16_MAX_LENGTH, srcChar, isError);
+  (void) isError;
   return doReplace(length(), 0, buffer, 0, _length);
 }