_clip_and_composite_trapezoids: Avoid NULL pointer dereference due to OOM.

This closes bug #6197:

	Null pointer dereference on OOM in _clip_and_composite_trapezoids()
	https://bugs.freedesktop.org/show_bug.cgi?id=6197
(cherry picked from 6f0d8eedc5 commit)

src/cairo-gstate.c

@@ -1439,6 +1439,9 @@ _cairo_surface_clip_and_composite_trapezoids (cairo_pattern_t *src,
 		return status;
 	    
 	    clear_region = _cairo_region_create_from_rectangle (&extents);
+	    if (clear_region == NULL)
+		return CAIRO_STATUS_NO_MEMORY;
+
 	    status = _cairo_clip_intersect_to_region (clip, clear_region);
 	    if (status)
 		return status;