avcodec/h264_cabac: Tighten allowed coeff_abs range

Fixes: integer overflows
Reported-by: "Xiaohan Wang (王消寒)" <xhwang@chromium.org>

Based on limits in "8.5 Transform coefficient decoding process and picture
construction process prior to deblocking  filter process"

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>