avcodec/bethsoftvideo: Check block_type

Fixes: Timeout (17 seconds -> 1 second)
Fixes: 13184/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_BETHSOFTVID_fuzzer-5711446296494080

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