iwlwifi: queue: don't crash if txq->entries is NULL

The code was really awkward, we would first dereference
txq->entries when calling iwl_txq_genX_tfd_unmap and then
we would check that txq->entries is non-NULL.
Fix that by exiting if txq->entries is NULL.
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.20210115130252.173359fc236d.I75c7c2397d20df8d7fbc24cb16a5232d5c551889@changeid

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

@@ -142,16 +142,16 @@ void iwl_txq_gen2_free_tfd(struct iwl_trans *trans, struct iwl_txq *txq)
 	 * idx is bounded by n_window
 	 */
 	int idx = iwl_txq_get_cmd_index(txq, txq->read_ptr);
+	struct sk_buff *skb;
 
 	lockdep_assert_held(&txq->lock);
 
+	if (!txq->entries)
+		return;
+
 	iwl_txq_gen2_tfd_unmap(trans, &txq->entries[idx].meta,
 			       iwl_txq_get_tfd(trans, txq, idx));
 
-	/* free SKB */
-	if (txq->entries) {
-		struct sk_buff *skb;
-
 	skb = txq->entries[idx].skb;
 
 	/* Can be called from irqs-disabled context
@@ -162,7 +162,6 @@ void iwl_txq_gen2_free_tfd(struct iwl_trans *trans, struct iwl_txq *txq)
 		iwl_op_mode_free_skb(trans->op_mode, skb);
 		txq->entries[idx].skb = NULL;
 	}
-	}
 }
 
 int iwl_txq_gen2_set_tb(struct iwl_trans *trans, struct iwl_tfh_tfd *tfd,
@@ -1494,18 +1493,19 @@ void iwl_txq_free_tfd(struct iwl_trans *trans, struct iwl_txq *txq)
 	 */
 	int rd_ptr = txq->read_ptr;
 	int idx = iwl_txq_get_cmd_index(txq, rd_ptr);
+	struct sk_buff *skb;
 
 	lockdep_assert_held(&txq->lock);
 
+	if (!txq->entries)
+		return;
+
 	/* We have only q->n_window txq->entries, but we use
 	 * TFD_QUEUE_SIZE_MAX tfds
 	 */
 	iwl_txq_gen1_tfd_unmap(trans, &txq->entries[idx].meta, txq, rd_ptr);
 
 	/* free SKB */
-	if (txq->entries) {
-		struct sk_buff *skb;
-
 	skb = txq->entries[idx].skb;
 
 	/* Can be called from irqs-disabled context
@@ -1516,7 +1516,6 @@ void iwl_txq_free_tfd(struct iwl_trans *trans, struct iwl_txq *txq)
 		iwl_op_mode_free_skb(trans->op_mode, skb);
 		txq->entries[idx].skb = NULL;
 	}
-	}
 }
 
 void iwl_txq_progress(struct iwl_txq *txq)