cliptest: Avoid use of uninitialized memory when regions don't intersect
cliptest: Avoid use of uninitialized memory when regions don't intersect

Prevent attempting to draw the intersection polygon when it contains no
vertices.

Signed-off-by: Derek Foreman <2f97e0f0bc172406aab8ccd42dab54c95bab8759@osg.samsung.com>
Reviewed-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>

giucam/weston,Gnurou/weston,kwm81/weston,eyolfson/weston,xorgy/weston,Fantu/compositor-spice,jonnylamb/weston,mchalupa/weston,Fantu/compositor-spice,jonnylamb/weston,eyolfson/weston,kwm81/weston,Tarnyko/weston-xdg_surface_present,udoprog/weston,krezovic/weston,xorgy/weston,udoprog/weston,Gnurou/weston,Fantu/compositor-spice,mchalupa/weston,Tarnyko/weston-xdg_surface_present,krezovic/weston,sir-murray/weston,giucam/weston,sir-murray/weston