compute_composite_region32: Zero extents before returning FALSE.

If the extents of the composite region are broken such that x2 <= x1
or y2 <= y1, then we need to zero the extents before returning so that
the region won't be completely broken when calling
pixman_region32_fini().
diff --git a/pixman/pixman.c b/pixman/pixman.c
index 2296325..bd9305b 100644
--- a/pixman/pixman.c
+++ b/pixman/pixman.c
@@ -306,6 +306,10 @@
     if (region->extents.x1 >= region->extents.x2 ||
         region->extents.y1 >= region->extents.y2)
     {
+	region->extents.x1 = 0;
+	region->extents.x2 = 0;
+	region->extents.y1 = 0;
+	region->extents.y2 = 0;
 	return FALSE;
     }