dvdsubdec.c: prevent input buffer overflow

In some places, dvbsubdec passes improper input buffer size to
bitstream reading functions, not accounting for reading pointer
updates.

Fixed by using buffer_end - buffer pointer instead of fixed buffer length.

Signed-off-by: Jindrich Makovicka <makovick@gmail.com>

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