Add the missing 'return' to QProtobufSerializerPrivate::serializeProperty

If we are confident that the property is message, we shouldn't attempt
to look for other serializers.

Pick-to: 6.7
Change-Id: I61c76d9dc2c43d5b8a91627d1e3a7aaebbb8d166
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/protobuf/qprotobufserializer.cpp

@@ -569,6 +569,7 @@ void QProtobufSerializerPrivate::serializeProperty(const QVariant &propertyValue
         store.append(QProtobufSerializerPrivate::serializeVarintCommon<uint32_t>(result.size()));
         store.append(result);
         result = store;
+        return;
     }
 
     auto basicHandler = findIntegratedTypeHandler(