avcodec/ylc: Check count in build_vlc()

Fixes: runtime error: signed integer overflow: 211633430 + 2147483647 cannot be represented in type 'int'
Fixes: 1874/clusterfuzz-testcase-minimized-5037763613163520

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