ffplay: if there is no audio stream, use external clock by default

Otherwise playing the video could be much slower than realtime if the system
can't decode or display the frames fast enough.
Signed-off-by: Marton Balint <cus@passwd.hu>

ffplay.c

@@ -1076,7 +1076,7 @@ static int get_master_sync_type(VideoState *is) {
         if (is->audio_st)
             return AV_SYNC_AUDIO_MASTER;
         else
-            return AV_SYNC_VIDEO_MASTER;
+            return AV_SYNC_EXTERNAL_CLOCK;
     } else {
         return AV_SYNC_EXTERNAL_CLOCK;
     }