avcodec/hevcdsp_template: Fix undefined shift in put_hevc_epel_bi_w_h()

Fixes: runtime error: left shift of negative value -127
Fixes: 4397/clusterfuzz-testcase-minimized-4779061080489984

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