Add NULL pointer checks in mt792x_tx() to prevent kernel crashes when
transmitting packets during MLO link removal.

The function calls mt792x_sta_to_link() which can return NULL if the
link is being removed, but the return value was dereferenced without
checking. Similarly, the RCU-protected link_conf and link_sta pointers
were used without NULL validation.

This race can occur when:
1. A packet is queued for transmission
2. Concurrently, the link is being removed (mt7925_mac_link_sta_remove)
3. mt792x_sta_to_link() returns NULL for the removed link
4. Kernel crashes on wcid = &mlink->wcid dereference

Example crash trace:
  BUG: kernel NULL pointer dereference
  RIP: mt792x_tx+0x...
  Call Trace:
   ieee80211_tx+0x...
   __ieee80211_subif_start_xmit+0x...

Fix by:
- Check mlink return value before dereferencing wcid
- Check RCU-dereferenced conf and link_sta before use
- Free the SKB and return early if any pointer is NULL

This affects both MT7921 and MT7925 drivers as mt792x_core.c is shared.

Fixes: c74df1c067f2 ("wifi: mt76: mt792x: introduce mt792x-lib module")
Reported-by: Zac Bowling <zac@zacbowling.com>
Signed-off-by: Zac Bowling <zac@zacbowling.com>
---
 drivers/net/wireless/mediatek/mt76/mt792x_core.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/net/wireless/mediatek/mt76/mt792x_core.c b/drivers/net/wireless/mediatek/mt76/mt792x_core.c
index f2ed16feb6c1..9dc768aa8b9c 100644
--- a/drivers/net/wireless/mediatek/mt76/mt792x_core.c
+++ b/drivers/net/wireless/mediatek/mt76/mt792x_core.c
@@ -95,6 +95,8 @@ void mt792x_tx(struct ieee80211_hw *hw, struct ieee80211_tx_control *control,
 				       IEEE80211_TX_CTRL_MLO_LINK);
 		sta = (struct mt792x_sta *)control->sta->drv_priv;
 		mlink = mt792x_sta_to_link(sta, link_id);
+		if (!mlink)
+			goto free_skb;
 		wcid = &mlink->wcid;
 	}
 
@@ -113,9 +115,12 @@ void mt792x_tx(struct ieee80211_hw *hw, struct ieee80211_tx_control *control,
 		link_id = wcid->link_id;
 		rcu_read_lock();
 		conf = rcu_dereference(vif->link_conf[link_id]);
-		memcpy(hdr->addr2, conf->addr, ETH_ALEN);
-
 		link_sta = rcu_dereference(control->sta->link[link_id]);
+		if (!conf || !link_sta) {
+			rcu_read_unlock();
+			goto free_skb;
+		}
+		memcpy(hdr->addr2, conf->addr, ETH_ALEN);
 		memcpy(hdr->addr1, link_sta->addr, ETH_ALEN);
 
 		if (vif->type == NL80211_IFTYPE_STATION)
@@ -136,6 +141,10 @@ void mt792x_tx(struct ieee80211_hw *hw, struct ieee80211_tx_control *control,
 	}
 
 	mt76_connac_pm_queue_skb(hw, &dev->pm, wcid, skb);
+	return;
+
+free_skb:
+	ieee80211_free_txskb(hw, skb);
 }
 EXPORT_SYMBOL_GPL(mt792x_tx);
 
-- 
2.51.0