AU Client: Only call prepareToPlay() from SetProperty() if already in prepared state

modules/juce_audio_plugin_client/AU/juce_AU_Wrapper.mm

@@ -633,7 +633,9 @@ public:
                             const ScopedLock sl (juceFilter->getCallbackLock());
 
                             juceFilter->setNonRealtime (shouldBeOffline);
-                            juceFilter->prepareToPlay (getSampleRate(), (int) GetMaxFramesPerSlice());
+
+                            if (prepared)
+                                juceFilter->prepareToPlay (getSampleRate(), (int) GetMaxFramesPerSlice());
                         }
                     }