avcodec/hevc_cabac: trivial simplifications

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/hevc: do not dereference pointer before null check in verify_md5()

Fixes: CID1108607

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/hevc_refs: fix h/vshift calculation in ff_hevc_output_frame()

Fixes CID1108577

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>