mxfdec: Fix integer overflow with many channels

Fixes division by zero

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Reviewed-by: Matthieu Bouron <matthieu.bouron@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mxfdec.c

@@ -2070,7 +2070,7 @@ static int mxf_set_audio_pts(MXFContext *mxf, AVCodecContext *codec, AVPacket *p
     pkt->pts = track->sample_count;
     if (codec->channels <= 0 || av_get_bits_per_sample(codec->codec_id) <= 0)
         return AVERROR(EINVAL);
-    track->sample_count += pkt->size / (codec->channels * av_get_bits_per_sample(codec->codec_id) / 8);
+    track->sample_count += pkt->size / (codec->channels * (int64_t)av_get_bits_per_sample(codec->codec_id) / 8);
     return 0;
 }