[cogl/cogl-1.22] winsys: glx: fix crash when inspecting onscreens

[Lionel Landwerlin <llandwerlin src gnome org>]

commit 00ea695ce26a23e84cd6e7a425be4f0d2221f7f2
Author: Lionel Landwerlin <llandwerlin gmail com>
Date:   Fri Sep 11 17:29:15 2015 +0200

    winsys: glx: fix crash when inspecting onscreens
    
    An unallocated onscreen might have a NULL winsys field.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=754888

 cogl/winsys/cogl-winsys-glx.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/cogl/winsys/cogl-winsys-glx.c b/cogl/winsys/cogl-winsys-glx.c
index 4413873..72d9e56 100644
--- a/cogl/winsys/cogl-winsys-glx.c
+++ b/cogl/winsys/cogl-winsys-glx.c
@@ -180,7 +180,7 @@ find_onscreen_for_xid (CoglContext *context, uint32_t xid)
 
       /* Does the GLXEvent have the GLXDrawable or the X Window? */
       xlib_onscreen = COGL_ONSCREEN (framebuffer)->winsys;
-      if (xlib_onscreen->xwin == (Window)xid)
+      if (xlib_onscreen != NULL && xlib_onscreen->xwin == (Window)xid)
         return COGL_ONSCREEN (framebuffer);
     }