avcodec/wavpack: Do not allow the sample format to change between channels

Fixes: out of array access
Fixes: 22692/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WAVPACK_fuzzer-5678686190960640

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

libavcodec/wavpack.c

@@ -1129,6 +1129,9 @@ static int wavpack_decode_block(AVCodecContext *avctx, int block_no,
     else
         sample_fmt          = AV_SAMPLE_FMT_S32P;
 
+    if (wc->ch_offset && avctx->sample_fmt != sample_fmt)
+        return AVERROR_INVALIDDATA;
+
     bpp            = av_get_bytes_per_sample(sample_fmt);
     orig_bpp       = ((s->frame_flags & 0x03) + 1) << 3;
     multiblock     = (s->frame_flags & WV_SINGLE_BLOCK) != WV_SINGLE_BLOCK;