The check `src > dst' in the form `&c->out[-back] > c->out' invokes
pointer overflow, which is undefined behavior in C.

Remove the check.  Also replace `&c->out[-back] < c->out_start' with
a safe form `c->out - c->out_start < back' to avoid overflow.

CC: libav-stable@libav.org
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>