Don't coredump on "X -showopts" (bug 25874)

Don't try walking the xf86ConfigLayout.screens table if it's empty
https://bugs.freedesktop.org/show_bug.cgi?id=25874

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Tiago Vignatti <tiago.vignatti@nokia.com>
Signed-off-by: Keith Packard <keithp@keithp.com>

hw/xfree86/common/xf86Helper.c

@@ -1440,6 +1440,13 @@ xf86MatchDevice(const char *drivername, GDevPtr **sectlist)
     if (sectlist)
 	*sectlist = NULL;
 
+    /*
+     * This can happen when running Xorg -showopts and a module like ati
+     * or vmware tries to load its submodules when xf86ConfigLayout is empty
+     */
+    if (!xf86ConfigLayout.screens)
+	return 0;
+
     /*
      * This is a very important function that matches the device sections
      * as they show up in the config file with the drivers that the server