QMetaEnum: stop parsing when an invalid key is found

The old code obfuscated the algorithm by or'ing -1 into the return
value, which is equivalent to setting it to -1 on two's complement
architectures (which C++ these days requires), and happily continuing
to accumulate potential keys. But nothing that can be or'ed into -1
will ever change the value, so this is rather pointless, as we're not
emitting diagnostics apart from setting a bool to false.

Fix by simply returning -1. That makes it obvious what we're
returning, and we return early on error.

Pick-to: 6.2
Change-Id: I8957f44e03609ad58d6c25d5fa78c57190b14bdd
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/kernel/qmetaobject.cpp

@@ -2872,7 +2872,7 @@ int QMetaEnum::keysToValue(const char *keys, bool *ok) const
         if (i < 0) {
             if (ok != nullptr)
                 *ok = false;
-            value |= -1;
+            return -1;
         }
     }
     return value;