avcodec/hevcdsp_template: Fix invalid shift in put_hevc_epel_bi_w_v()

Fixes: runtime error: left shift of negative value -255
Fixes: 4037/clusterfuzz-testcase-minimized-5290998163832832

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