wifi: mt76: mt7925: prevent NULL vif dereference in mt7925_mac_write_txwi

[ Upstream commit 962eb04 ]

Check for a NULL `vif` before accessing `ieee80211_vif_is_mld(vif)` to
avoid a potential kernel panic in scenarios where `vif` might not be
initialized.

Fixes: ebb1406 ("wifi: mt76: mt7925: add link handling to txwi")
Signed-off-by: Ming Yen Hsieh <mingyen.hsieh@mediatek.com>
Link: https://patch.msgid.link/20250904030649.655436-3-mingyen.hsieh@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: Ming Yen Hsieh

drivers/net/wireless/mediatek/mt76/mt7925/mac.c

@@ -803,8 +803,8 @@ mt7925_mac_write_txwi(struct mt76_dev *dev, __le32 *txwi,
 	txwi[5] = cpu_to_le32(val);
 
 	val = MT_TXD6_DAS | FIELD_PREP(MT_TXD6_MSDU_CNT, 1);
-	if (!ieee80211_vif_is_mld(vif) ||
-	    (q_idx >= MT_LMAC_ALTX0 && q_idx <= MT_LMAC_BCN0))
+	if (vif && (!ieee80211_vif_is_mld(vif) ||
+	    (q_idx >= MT_LMAC_ALTX0 && q_idx <= MT_LMAC_BCN0)))
 		val |= MT_TXD6_DIS_MAT;
 	txwi[6] = cpu_to_le32(val);
 	txwi[7] = 0;