Revert "audiounit_ios: try to handle data drops more efficiently"

This (more or less) reverts commit
2b7e0168.

Stopping the aout from within the callback like this could
lead to deadlocks, where AudioOutputUnitStop in the callback thread
and AudioOutputUnitStart in the audio decoder thread blocked each
other (noticed at startup of playback on a 3GS with iOS 6.0).
Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

modules/audio_output/audiounit_ios.c

@@ -383,8 +383,8 @@ static OSStatus RenderCallback(vlc_object_t *p_obj,
 
     /* check if we have enough data */
     if (!availableBytes) {
-        /* bail out and restart unit the next time we receive some data */
-        Flush(p_aout, false);
+        /* return an empty buffer so silence is played until we have data */
+        memset(targetBuffer, 0, bytesToCopy);
     } else {
         memcpy(targetBuffer, buffer, __MIN(bytesToCopy, availableBytes));
         TPCircularBufferConsume(&p_sys->circular_buffer, __MIN(bytesToCopy, availableBytes));