get_bits: change the failure condition in init_get_bits

Too much code relies in having init_get_bits fed with a valid
buffer and set its dimension to 0.

Check for NULL buffer instead.
(cherry picked from commit 4603ec85ed620e585fc6e2e072c99858ed421855)

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