iwlwifi: warn on SKB free w/o op-mode

If this ever happens, and it looks like some code in PCIe is
a bit broken and might lead to this, we want to know without
crashing, so add a WARN_ON_ONCE().
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210411124418.a18b7443dc55.Ia29836738acf14a55af5504aba90c6fea9fff785@changeidSigned-off-by: Luca Coelho <luciano.coelho@intel.com>

drivers/net/wireless/intel/iwlwifi/iwl-op-mode.h

 /* SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause */
 /*
- * Copyright (C) 2005-2014, 2018-2020 Intel Corporation
+ * Copyright (C) 2005-2014, 2018-2021 Intel Corporation
  * Copyright (C) 2013-2014 Intel Mobile Communications GmbH
  * Copyright (C) 2015 Intel Deutschland GmbH
  */
@@ -176,6 +176,8 @@ iwl_op_mode_hw_rf_kill(struct iwl_op_mode *op_mode, bool state)
 static inline void iwl_op_mode_free_skb(struct iwl_op_mode *op_mode,
 					struct sk_buff *skb)
 {
+	if (WARN_ON_ONCE(!op_mode))
+		return;
 	op_mode->ops->free_skb(op_mode, skb);
 }