avformat/apm: Check channels

Fixes: division by 0
Fixes: 45643/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-4957777905188864.fuzz

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/apm.c

@@ -140,7 +140,7 @@ static int apm_read_header(AVFormatContext *s)
     if (par->bits_per_coded_sample != 4)
         return AVERROR_INVALIDDATA;
 
-    if (channels > 2)
+    if (channels > 2 || channels == 0)
         return AVERROR_INVALIDDATA;
 
     av_channel_layout_default(&par->ch_layout, channels);