avcodec/wavpack: Treat the first block coding too many channels as an error

Fixes memleak
Fixes: 236/8aeebc9ca49b91bf71c114dcefac56c154a3a563

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