avcodec/get_bits: add BITS_LEFT() for finding the bits left with an opened reader

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/huffyuvdec: fix overread checks

Fixes: ffvhuff_f.avi
Found-by: Piotr Bandurski <ami_stuff@o2.pl>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>