avcodec/dstdec: Check sample rate

Fixes: out of array access
Fixes: 20659/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DST_fuzzer-5735812071424000

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

libavcodec/dstdec.c

@@ -86,6 +86,10 @@ static av_cold int decode_init(AVCodecContext *avctx)
         return AVERROR_PATCHWELCOME;
     }
 
+    if (DST_SAMPLES_PER_FRAME(avctx->sample_rate) & 7) {
+        return AVERROR_PATCHWELCOME;
+    }
+
     avctx->sample_fmt = AV_SAMPLE_FMT_FLT;
 
     for (i = 0; i < avctx->channels; i++)