dix: block signals when closing all devices

When closing down all devices, we manually unset master for all attached
devices, but the device's sprite info still points to the master's sprite
info. This leaves us a window where the master is freed already but the
device isn't yet. A signal during that window causes dereference of the
already freed spriteInfo in mieqEnqueue's EnqueueScreen macro.

Simply block signals when removing all devices. It's not like we're really
worrying about high-responsive input at this stage.

https://bugzilla.redhat.com/show_bug.cgi?id=737031

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Julien Cristau <jcristau@debian.org>

dix/devices.c

@@ -985,6 +985,8 @@ CloseDownDevices(void)
 {
     DeviceIntPtr dev;
 
+    OsBlockSignals();
+
     /* Float all SDs before closing them. Note that at this point resources
      * (e.g. cursors) have been freed already, so we can't just call
      * AttachDevice(NULL, dev, NULL). Instead, we have to forcibly set master
@@ -1007,6 +1009,8 @@ CloseDownDevices(void)
     inputInfo.keyboard = NULL;
     inputInfo.pointer = NULL;
     XkbDeleteRulesDflts();
+
+    OsReleaseSignals();
 }
 
 /**