lavf/wavdec: add sanity check for AVCodecContext.channels

Fixes ticket #3862.
As a side effect, this also fixes aac_latm in wav.
Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/wavdec.c

@@ -402,7 +402,8 @@ break_loop:
 
     avio_seek(pb, data_ofs, SEEK_SET);
 
-    if (data_size > 0 && sample_count && data_size / sample_count / st->codec->channels > 8) {
+    if (   data_size > 0 && sample_count && st->codec->channels
+        && data_size / sample_count / st->codec->channels > 8) {
         av_log(s, AV_LOG_WARNING, "ignoring wrong sample_count %"PRId64"\n", sample_count);
         sample_count = 0;
     }