avcodec/hevcpred_template: Fix left shift of negative value

Fixes: runtime error: left shift of negative value -1
Fixes: 2250/clusterfuzz-testcase-minimized-5693382112313344

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