iwlwifi: mvm: Check the sta is not NULL in iwl_mvm_cfg_he_sta()

Fix a kernel panic by checking that the sta is not NULL.
This could happen during a reconfig flow, as mac80211 moves the sta
between all the states without really checking if the previous state was
successfully set. So, if for some reason we failed to add back the
station, subsequent calls to sta_state() callback will be done when the
station is NULL. This would result in a following panic:

BUG: unable to handle kernel NULL pointer dereference at
0000000000000040
IP: iwl_mvm_cfg_he_sta+0xfc/0x690 [iwlmvm]
[..]
Call Trace:
 iwl_mvm_mac_sta_state+0x629/0x6f0 [iwlmvm]
 drv_sta_state+0xf4/0x950 [mac80211]
 ieee80211_reconfig+0xa12/0x2180 [mac80211]
 ieee80211_restart_work+0xbb/0xe0 [mac80211]
 process_one_work+0x1e2/0x610
 worker_thread+0x4d/0x3e0
[..]
Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/intel/iwlwifi/mvm/mac80211.c

@@ -5,10 +5,9 @@
  *
  * GPL LICENSE SUMMARY
  *
- * Copyright(c) 2012 - 2014 Intel Corporation. All rights reserved.
  * Copyright(c) 2013 - 2015 Intel Mobile Communications GmbH
  * Copyright(c) 2016 - 2017 Intel Deutschland GmbH
- * Copyright(c) 2018 - 2019 Intel Corporation
+ * Copyright(c) 2012 - 2014, 2018 - 2020 Intel Corporation
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of version 2 of the GNU General Public License as
@@ -28,10 +27,9 @@
  *
  * BSD LICENSE
  *
- * Copyright(c) 2012 - 2014 Intel Corporation. All rights reserved.
  * Copyright(c) 2013 - 2015 Intel Mobile Communications GmbH
  * Copyright(c) 2016 - 2017 Intel Deutschland GmbH
- * Copyright(c) 2018 - 2019 Intel Corporation
+ * Copyright(c) 2012 - 2014, 2018 - 2020 Intel Corporation
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -2037,7 +2035,7 @@ static void iwl_mvm_cfg_he_sta(struct iwl_mvm *mvm,
 	rcu_read_lock();
 
 	sta = rcu_dereference(mvm->fw_id_to_mac_id[sta_ctxt_cmd.sta_id]);
-	if (IS_ERR(sta)) {
+	if (IS_ERR_OR_NULL(sta)) {
 		rcu_read_unlock();
 		WARN(1, "Can't find STA to configure HE\n");
 		return;