avcodec/hevc_cabac: Check prefix so as to avoid invalid shifts in coeff_abs_level_remaining_decode()

I suspect that this can be limited tighter, but i failed to find anything
in the spec that would confirm that.

Fixes: 4833/clusterfuzz-testcase-minimized-5302840101699584
Fixes: runtime error: left shift of 134217730 by 4 places cannot be represented in type 'int'

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