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/ffmpegReviewed-by: David Bryant <david@wavpack.com>
Tested-by: David Bryant <david@wavpack.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>