avformat/soxdec: Check channels to be positive

Fixes: signed integer overflow: 32 * -1795162112 cannot be represented in type 'int'
Fixes: 26910/clusterfuzz-testcase-minimized-ffmpeg_dem_SOX_fuzzer-6724151473340416

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

libavformat/soxdec.c

@@ -90,7 +90,7 @@ static int sox_read_header(AVFormatContext *s)
                sample_rate_frac);
 
     if ((header_size + 4) & 7 || header_size < SOX_FIXED_HDR + comment_size
-        || st->codecpar->channels > 65535) /* Reserve top 16 bits */ {
+        || st->codecpar->channels > 65535 || st->codecpar->channels <= 0) /* Reserve top 16 bits */ {
         av_log(s, AV_LOG_ERROR, "invalid header\n");
         return AVERROR_INVALIDDATA;
     }