xen pvfb: Zero unused bytes in events sent to backend

This isn't a security flaw (the backend can see all our memory
anyway).  But it's the right thing to do all the same.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

drivers/video/xen-fbfront.c

@@ -61,6 +61,7 @@ static void xenfb_do_update(struct xenfb_info *info,
 	union xenfb_out_event event;
 	u32 prod;
 
+	memset(&event, 0, sizeof(event));
 	event.type = XENFB_TYPE_UPDATE;
 	event.update.x = x;
 	event.update.y = y;