wl1271: Check vif for NULL when indicating beacon-loss

Because the interface is started and the vif are created and destroyed
separately, there is a slim possibility beacon-loss indications occur while
there is no vif - causing a kernel-oops unless checked.

Add checking for the vif.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/wl12xx/wl1271_event.c

@@ -150,7 +150,7 @@ static int wl1271_event_process(struct wl1271 *wl, struct event_mailbox *mbox)
 			return ret;
 	}
 
-	if (beacon_loss) {
+	if (wl->vif && beacon_loss) {
 		/* Obviously, it's dangerous to release the mutex while
 		   we are holding many of the variables in the wl struct.
 		   That's why it's done last in the function, and care must