avformat/movenc: fix writing dOps atoms

Don't blindly copy all bytes in extradata past ChannelMappingFamily. Instead
check if ChannelMappingFamily is not 0 and then only write the correct amount
of bytes from ChannelMappingTable, as defined in the spec[1].

Fixes part of ticket #9190.

[1] https://opus-codec.org/docs/opus_in_isobmff.html#4.3.2

Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/movenc.c

@@ -797,6 +797,7 @@ static int mov_write_dfla_tag(AVIOContext *pb, MOVTrack *track)
 static int mov_write_dops_tag(AVFormatContext *s, AVIOContext *pb, MOVTrack *track)
 {
     int64_t pos = avio_tell(pb);
+    int channels, channel_map;
     avio_wb32(pb, 0);
     ffio_wfourcc(pb, "dOps");
     avio_w8(pb, 0); /* Version */
@@ -807,12 +808,22 @@ static int mov_write_dops_tag(AVFormatContext *s, AVIOContext *pb, MOVTrack *tra
     /* extradata contains an Ogg OpusHead, other than byte-ordering and
        OpusHead's preceeding magic/version, OpusSpecificBox is currently
        identical. */
-    avio_w8(pb, AV_RB8(track->par->extradata + 9)); /* OuputChannelCount */
+    channels = AV_RB8(track->par->extradata + 9);
+    channel_map = AV_RB8(track->par->extradata + 18);
+
+    avio_w8(pb, channels); /* OuputChannelCount */
     avio_wb16(pb, AV_RL16(track->par->extradata + 10)); /* PreSkip */
     avio_wb32(pb, AV_RL32(track->par->extradata + 12)); /* InputSampleRate */
     avio_wb16(pb, AV_RL16(track->par->extradata + 16)); /* OutputGain */
+    avio_w8(pb, channel_map); /* ChannelMappingFamily */
     /* Write the rest of the header out without byte-swapping. */
-    avio_write(pb, track->par->extradata + 18, track->par->extradata_size - 18);
+    if (channel_map) {
+        if (track->par->extradata_size < 21 + channels) {
+            av_log(s, AV_LOG_ERROR, "invalid extradata size\n");
+            return AVERROR_INVALIDDATA;
+        }
+        avio_write(pb, track->par->extradata + 19, 2 + channels); /* ChannelMappingTable */
+    }
 
     return update_size(pb, pos);
 }