wlcore: fix unsafe dereference of the wlvif

wlvif could be passed as NULL from the wlcore_tx_work_locked()
to the wl1271_prepare_tx_frame() and to wl1271_skb_queue_head()
functions. This may lead to a Kernel panic, fix this by
validating that wlvif != NULL.
Signed-off-by: Victor Goldenshtein <victorg@ti.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>

drivers/net/wireless/ti/wlcore/tx.c

@@ -401,7 +401,7 @@ static int wl1271_prepare_tx_frame(struct wl1271 *wl, struct wl12xx_vif *wlvif,
 		is_wep = (cipher == WLAN_CIPHER_SUITE_WEP40) ||
 			 (cipher == WLAN_CIPHER_SUITE_WEP104);
 
-		if (WARN_ON(is_wep && wlvif->default_key != idx)) {
+		if (WARN_ON(is_wep && wlvif && wlvif->default_key != idx)) {
 			ret = wl1271_set_default_wep_key(wl, wlvif, idx);
 			if (ret < 0)
 				return ret;