iwlwifi: pcie: add a NULL check in iwl_pcie_txq_unmap

I hit a NULL pointer exception in this function when the
init flow went really bad.
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20210115130252.2e8da9f2c132.I0234d4b8ddaf70aaa5028a20c863255e05bc1f84@changeid

drivers/net/wireless/intel/iwlwifi/pcie/tx.c

@@ -201,6 +201,11 @@ static void iwl_pcie_txq_unmap(struct iwl_trans *trans, int txq_id)
 	struct iwl_trans_pcie *trans_pcie = IWL_TRANS_GET_PCIE_TRANS(trans);
 	struct iwl_txq *txq = trans->txqs.txq[txq_id];
 
+	if (!txq) {
+		IWL_ERR(trans, "Trying to free a queue that wasn't allocated?\n");
+		return;
+	}
+
 	spin_lock_bh(&txq->lock);
 	while (txq->write_ptr != txq->read_ptr) {
 		IWL_DEBUG_TX_REPLY(trans, "Q %d Free %d\n",