Fix tautological compare in error checking

size is size_t, so it's never less than zero.

Fixes: QTBUG-72286
Change-Id: Idd0c85a4e7b64f9c9c7dfffd156d404d0de5ed8d
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/codecs/qwindowscodec.cpp

@@ -179,7 +179,7 @@ QString QWindowsLocalCodec::convertToUnicodeCharByChar(const char *chars, int le
 #else
     QString s;
     size_t size = mbstowcs(NULL, mb, length);
-    if (size < 0) {
+    if (size == size_t(-1)) {
         Q_ASSERT("Error in CE TextCodec");
         return QString();
     }