mov_chan: Only set the channel_layout if setting it to a nonzero value

If regularly parsing new chan atoms (as in rtpdec_qt), but the
chan atoms don't actually contain any channel layout, don't reset
the value that the caller has filled in (by guessing).
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/mov_chan.c

@@ -579,9 +579,10 @@ int ff_mov_read_chan(AVFormatContext *s, AVIOContext *pb, AVStream *st,
             label_mask |= mask_incr;
         }
     }
-    if (layout_tag == 0)
+    if (layout_tag == 0) {
+        if (label_mask)
             st->codec->channel_layout = label_mask;
-    else
+    } else
         st->codec->channel_layout = ff_mov_get_channel_layout(layout_tag, bitmap);
 
     return 0;