avcodec/utils: Require a non zero channels unless AV_CODEC_CAP_CHANNEL_CONF is set

This avoids per codec checks for channels not being 0

Fixes: division by 0
Fixes: 25419/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FASTAUDIO_fuzzer-5632544761184256
Fixes: 25433/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FASTAUDIO_fuzzer-6215671900536832

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSuggested-by: NPaul B Mahol <onemda@gmail.com>
See: [FFmpeg-devel] [PATCH 1/3] avcodec/fastaudio: Check channel
Reviewed-by: NPaul B Mahol <onemda@gmail.com>
Signed-off-by: NMichael Niedermayer <michael@niedermayer.cc>

libavcodec/utils.c

@@ -667,6 +667,15 @@ int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *code
         ret = AVERROR(EINVAL);
         goto free_and_end;
     }
+    if (av_codec_is_decoder(codec) &&
+        codec->type == AVMEDIA_TYPE_AUDIO &&
+        !(codec->capabilities & AV_CODEC_CAP_CHANNEL_CONF) &&
+        avctx->channels == 0) {
+        av_log(avctx, AV_LOG_ERROR, "Decoder requires channel count but channels not set\n");
+        ret = AVERROR(EINVAL);
+        goto free_and_end;
+    }
+
     if (avctx->sample_rate < 0) {
         av_log(avctx, AV_LOG_ERROR, "Invalid sample rate: %d\n", avctx->sample_rate);
         ret = AVERROR(EINVAL);