mplayer: fix potential issue when ao_play() fails

ao_play() can fail; in that case a negative error code is returned.
This error code is returned by write_to_ao() in turn. The function
fill_audio_out_buffers(), which calls write_to_ao(), doesn't check for
any error codes, and will likely trigger the assertion following the
function call. Change write_to_ao() to return 0 on failure to hopefully
prevent crashes when AOs fail.

core/mplayer.c

@@ -2076,8 +2076,9 @@ static int write_to_ao(struct MPContext *mpctx, void *data, int len, int flags,
         // Keep correct pts for remaining data - could be used to flush
         // remaining buffer when closing ao.
         ao->pts += played / bps;
+        return played;
     }
-    return played;
+    return 0;
 }
 
 #define ASYNC_PLAY_DONE -3