avformat/mov_chan: never override number of channels based on chan atom

The channel designation metadata should not override the number of channels.
Let's warn the user if it is inconsistent, and keep the channel layout
unspecified.

Before the conversion to the channel layout API the code only set the mask, but
never overridden the channel count, so this restores the old behaviour.

Signed-off-by: Marton Balint <cus@passwd.hu>
(cherry picked from commit dc9d64f794)

libavformat/mov_chan.c

@@ -544,8 +544,13 @@ int ff_mov_read_chan(AVFormatContext *s, AVIOContext *pb, AVStream *st,
         mask = mov_get_channel_layout(layout_tag, bitmap);
 
     if (mask) {
-        av_channel_layout_uninit(&st->codecpar->ch_layout);
-        av_channel_layout_from_mask(&st->codecpar->ch_layout, mask);
+        if (!st->codecpar->ch_layout.nb_channels || av_popcount64(mask) == st->codecpar->ch_layout.nb_channels) {
+            av_channel_layout_uninit(&st->codecpar->ch_layout);
+            av_channel_layout_from_mask(&st->codecpar->ch_layout, mask);
+        } else {
+            av_log(s, AV_LOG_WARNING, "ignoring channel layout with %d channels because the real number of channels is %d\n",
+                   av_popcount64(mask), st->codecpar->ch_layout.nb_channels);
+        }
     }
     avio_skip(pb, size - 12);