avcodec/hevcdec: check ff_init_cabac_decoder() for failure

Fixes: runtime error: left shift of 1965559808 by 4 places cannot be represented in type 'int'
Fixes: 2333/clusterfuzz-testcase-minimized-5223935677300736

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