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 <derekf@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

clients/cliptest.c

@@ -570,6 +570,7 @@ draw_geometry(cairo_t *cr, struct weston_surface *surface,
 	cairo_set_source_rgba(cr, 0.0, 0.0, 1.0, 0.4);
 	cairo_fill(cr);
 
+	if (n) {
 		draw_polygon_closed(cr, ex, ey, n);
 		cairo_set_source_rgb(cr, 0.0, 1.0, 0.0);
 		cairo_stroke(cr);
@@ -577,6 +578,7 @@ draw_geometry(cairo_t *cr, struct weston_surface *surface,
 		cairo_set_source_rgba(cr, 0.0, 1.0, 0.0, 0.5);
 		draw_polygon_labels(cr, ex, ey, n);
 	}
+}
 
 static void
 redraw_handler(struct widget *widget, void *data)