As reported by Coverity, there's a potential risk of a division
by zero on some calls to jpeg_set_qual(), if quality is zero.

As quality can't be 0 or lower than that, adds an extra clause
to cover this special case.

Coverity reports: CID#11922280, CID#11922293, CID#11922295
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>