[metacity] compositor: don't crash when xregion is None

[Alberts Muktupāvels <muktupavels src gnome org>]

commit 927faedbab4da30d75c514179c5f2190d42e71ff
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Sun Apr 17 17:57:43 2016 +0300

    compositor: don't crash when xregion is None

 src/compositor/compositor-xrender.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/src/compositor/compositor-xrender.c b/src/compositor/compositor-xrender.c
index e786731..a7d33e6 100644
--- a/src/compositor/compositor-xrender.c
+++ b/src/compositor/compositor-xrender.c
@@ -623,6 +623,9 @@ xserver_region_to_cairo_region (Display       *xdisplay,
   int i;
   cairo_region_t *region;
 
+  if (xregion == None)
+    return NULL;
+
   xrects = XFixesFetchRegion (xdisplay, xregion, &nrects);
   if (xrects == NULL)
     return NULL;