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

Some lines of code taken from  0c5f8396:libavcodec/x86/cabac.h
and                            0c5f8396:libavcodec/cabac_functions.h
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>