xsub: feed init_get_bits the whole buffer

Do not use rlelen field for buffer size in init_get_bits, it is
only the size of the data for the first field.
Since it is not reliable, just use the size of the whole buffer.

Additional comments add removal of unused rlelen variable by
Reimar Döffinger.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>