As reported by Peter Kovar, there's a potential risk of a division by
zero on calls to jpeg_set_qual() when quality is zero.

As quality can't be 0 or lower than that, add an extra clause to cover
this special case.
Signed-off-by: Luis de Bethencourt <luis.bg@samsung.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>