audio-mode: don't change the mode unless there is no active conference

If a new conference is joined and as a result the current one is terminated,
don't ever attempt to set the audio mode.

react/features/mobile/audio-mode/middleware.js

     CONFERENCE_FAILED,
     CONFERENCE_LEFT,
     CONFERENCE_JOINED,
-    SET_AUDIO_ONLY
+    SET_AUDIO_ONLY,
+    getCurrentConference
 } from '../../base/conference';
 import { MiddlewareRegistry } from '../../base/redux';
 
         switch (action.type) {
         case APP_WILL_MOUNT:
         case CONFERENCE_FAILED:
-        case CONFERENCE_LEFT:
-            mode = AudioMode.DEFAULT;
+        case CONFERENCE_LEFT: {
+            const conference = getCurrentConference(getState());
+
+            if (typeof conference === 'undefined') {
+                mode = AudioMode.DEFAULT;
+            }
+
             break;
+        }
 
         /*
          * NOTE: We moved the audio mode setting from CONFERENCE_WILL_JOIN to