QmlJS: Synchronize Engine::registerValue.

When multiple threads share the Bind instance in a Document and do
lookup on the objects contained inside, that may trigger a new object
creation in some situations - which needs to be synchronized.

src/libs/qmljs/qmljsinterpreter.cpp

@@ -2528,6 +2528,7 @@ const ObjectValue *Engine::qtObject() const
 
 void Engine::registerValue(Value *value)
 {
+    QMutexLocker locker(&_mutex);
     _registeredValues.append(value);
 }