avcodec/hevcdsp_template.c: Fix undefined shift in FUNC(dequant)

Fixes: runtime error: left shift of negative value -180
Fixes: 4626/clusterfuzz-testcase-minimized-5647837887987712

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