wifi: zd1211rw: Fix potential NULL pointer dereference in zd_mac_tx_to_dev()

wifi: zd1211rw: Fix potential NULL pointer dereference in zd_mac_tx_to_dev()

There is a potential NULL pointer dereference in zd_mac_tx_to_dev(). For
example, the following is possible:

    	T0			    		T1
zd_mac_tx_to_dev()
  /* len == skb_queue_len(q) */
  while (len > ZD_MAC_MAX_ACK_WAITERS) {

					  filter_ack()
					    spin_lock_irqsave(&q->lock, flags);
					    /* position == skb_queue_len(q) */
					    for (i=1; i<position; i++)
				    	      skb = __skb_dequeue(q)

					    if (mac->type == NL80211_IFTYPE_AP)
					      skb = __skb_dequeue(q);
					    spin_unlock_irqrestore(&q->lock, flags);

    skb_dequeue() -> NULL

Since there is a small gap between checking skb queue length and skb being
unconditionally dequeued in zd_mac_tx_to_dev(), skb_dequeue() can return NULL.
Then the pointer is passed to zd_mac_tx_status() where it is dereferenced.

In order to avoid potential NULL pointer dereference due to situations like
above, check if skb is not NULL before passing it to zd_mac_tx_status().

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 459c51ad6e1f ("zd1211rw: port to mac80211")
Signed-off-by: Daniil Dulov <d.dulov@aladdin.ru>
Link: https://patch.msgid.link/20250626114619.172631-1-d.dulov@aladdin.ru
Signed-off-by: Johannes Berg <johannes.berg@intel.com>