avcodec/hevc_filter: Fix invalid shift

Fixes: runtime error: left shift of negative value -1

Fixes: 2299/clusterfuzz-testcase-minimized-4843509351710720

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