Fix incorrect warning in QML AudioEngine.

We need to do an early return when an AudioCategory is successfully
added to an AudioEngine, otherwise a warning is incorrectly shown.

Change-Id: If310c694a703242aff7f1c5ae04ad3e40c3f1acd
Reviewed-by: Christian Stromme <christian.stromme@theqtcompany.com>

src/imports/audioengine/qdeclarative_audioengine_p.cpp

@@ -373,6 +373,7 @@ void QDeclarativeAudioEngine::appendFunction(QQmlListProperty<QObject> *property
         if (category->name() == QLatin1String("default")) {
             engine->m_defaultCategory = category;
         }
+        return;
     }
 
     QDeclarativeAttenuationModel *attenModel = qobject_cast<QDeclarativeAttenuationModel*>(value);