mt76x2: fix possible NULL pointer dereferencing in mt76x2_tx()

Fix a theoretical NULL pointer dereferencing in mt76x2_tx routine that
can occurs for injected frames in a monitor vif since vif pointer could
be NULL for that interfaces

Fixes: 23405236460b ("mt76: fix transmission of encrypted mgmt frames")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Acked-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/mediatek/mt76/mt76x2_tx.c

@@ -36,9 +36,12 @@ void mt76x2_tx(struct ieee80211_hw *hw, struct ieee80211_tx_control *control,
 
 		msta = (struct mt76x2_sta *) control->sta->drv_priv;
 		wcid = &msta->wcid;
+		/* sw encrypted frames */
+		if (!info->control.hw_key && wcid->hw_key_idx != -1)
+			control->sta = NULL;
 	}
 
-	if (vif || (!info->control.hw_key && wcid->hw_key_idx != -1)) {
+	if (vif && !control->sta) {
 		struct mt76x2_vif *mvif;
 
 		mvif = (struct mt76x2_vif *) vif->drv_priv;