wifi: mt76: mt7925: add NULL checks for MLO link operations

Add NULL pointer checks for mt792x_sta_to_link() and mt792x_vif_to_link()
results in critical paths to prevent kernel crashes during MLO operations.

Found by static analysis review.

Author: zbowling

mt7925/main.c

@@ -856,12 +856,17 @@ static int mt7925_mac_link_sta_add(struct mt76_dev *mdev,
 
 	msta = (struct mt792x_sta *)link_sta->sta->drv_priv;
 	mlink = mt792x_sta_to_link(msta, link_id);
+	if (!mlink)
+		return -EINVAL;
 
 	idx = mt76_wcid_alloc(dev->mt76.wcid_mask, MT792x_WTBL_STA - 1);
 	if (idx < 0)
 		return -ENOSPC;
 
 	mconf = mt792x_vif_to_link(mvif, link_id);
+	if (!mconf)
+		return -EINVAL;
+
 	mt76_wcid_init(&mlink->wcid, 0);
 	mlink->wcid.sta = 1;
 	mlink->wcid.idx = idx;
@@ -1691,6 +1696,9 @@ mt7925_conf_tx(struct ieee80211_hw *hw, struct ieee80211_vif *vif,
 		    [IEEE80211_AC_BK] = 1,
 	};
 
+	if (!mconf)
+		return -EINVAL;
+
 	/* firmware uses access class index */
 	mconf->queue_params[mq_to_aci[queue]] = *params;