avcodec/dsddec: Check channels

Fixes: division by zero
Fixes: 21677/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DSD_MSBF_fuzzer-5712547983654912
Fixes: 21751/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DSD_LSBF_fuzzer-5197097180856320

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

libavcodec/dsddec.c

@@ -44,6 +44,9 @@ static av_cold int decode_init(AVCodecContext *avctx)
     int i;
     uint8_t silence;
 
+    if (!avctx->channels)
+        return AVERROR_INVALIDDATA;
+
     ff_init_dsd_data();
 
     s = av_malloc_array(sizeof(DSDContext), avctx->channels);