maybe fix audio backend not changing post-failure

src/engine/engine.cpp

@@ -6144,6 +6144,7 @@ bool DivEngine::initAudioBackend() {
     logE("error while initializing audio!\n");
     delete output;
     output=NULL;
+    audioEngine=DIV_AUDIO_NULL;
     return false;
   }
 
@@ -6155,6 +6156,7 @@ bool DivEngine::deinitAudioBackend() {
     output->quit();
     delete output;
     output=NULL;
+    audioEngine=DIV_AUDIO_NULL;
   }
   return true;
 }