avcodec/hevcdsp_template: Fix undefined shift

Fixes: runtime error: left shift of negative value -255
Fixes: 3373/clusterfuzz-testcase-minimized-5604083912146944

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