Bug 1732208 - Silence the unused-but-set-variable warning in parser. r=hsivonen

parser/html/nsHtml5StreamParser.cpp:1046:10: error: variable 'totalRead' set but not used [-Werror,-Wunused-but-set-variable]
  size_t totalRead = 0;
         ^

Differential Revision: https://phabricator.services.mozilla.com/D126458

parser/html/nsHtml5StreamParser.cpp

@@ -1076,6 +1076,7 @@ nsresult nsHtml5StreamParser::WriteStreamBytes(
     } else {
       MOZ_ASSERT(totalRead == aFromSegment.Length(),
                  "The Unicode decoder consumed the wrong number of bytes.");
+      (void)totalRead;
       if (mDecodingLocalFileWithoutTokenizing &&
           mLocalFileBytesBuffered == LOCAL_FILE_UTF_8_BUFFER_SIZE) {
         auto encoding = mEncoding;