xmv: check channel number

Fixes FPE

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/xmv.c

@@ -183,6 +183,11 @@ static int xmv_read_header(AVFormatContext *s)
         packet->bits_per_sample = avio_rl16(pb);
         packet->flags           = avio_rl16(pb);
 
+        if (!packet->channels) {
+            av_log(s, AV_LOG_ERROR, "0 channels\n");
+            return AVERROR(EINVAL);
+        }
+
         packet->bit_rate      = packet->bits_per_sample *
                                 packet->sample_rate *
                                 packet->channels;