avcodec/kgv1dec: Check that there is enough input for maximum RLE compression

Fixes: Timeout
Fixes: 4271/clusterfuzz-testcase-4676667768307712

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