avcodec/cabac: support UNCHECKED_BITSTREAM_READER = 0

Fixes overreads in HEVC
Fixes Ticket3070
Also fixed remaining issues from Ticket3075 and Ticket3076

Some lines of code taken from  0c5f839693da2276c2da23400f67a67be4ea0af1:libavcodec/x86/cabac.h
and                            0c5f839693da2276c2da23400f67a67be4ea0af1:libavcodec/cabac_functions.h

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