cw1200: Fix OOPS in monitor mode

In monitor mode, priv->vif is NULL, but at one point in the receive path we
blindly attempt to dereference it.  Add a test to prevent this.
Signed-off-by: Solomon Peachy <pizza@shaftnet.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/cw1200/txrx.c

@@ -1165,7 +1165,7 @@ void cw1200_rx_cb(struct cw1200_common *priv,
 		if (cw1200_handle_action_rx(priv, skb))
 			return;
 	} else if (ieee80211_is_beacon(frame->frame_control) &&
-		   !arg->status &&
+		   !arg->status && priv->vif &&
 		   !memcmp(ieee80211_get_SA(frame), priv->vif->bss_conf.bssid,
 			   ETH_ALEN)) {
 		const u8 *tim_ie;