wifi: mt76: handle failure of vzalloc in mt7615_coredump_work

vzalloc may fails, dump might be null and will cause
illegal address access later.

Link: https://lore.kernel.org/all/Y%2Fy5Asxw3T3m4jCw@lore-desk
Fixes: d2bf7959 ("mt76: mt7663: introduce coredump support")
Signed-off-by: Kang Chen <void0red@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7615/mac.c

@@ -2380,7 +2380,7 @@ void mt7615_coredump_work(struct work_struct *work)
 			break;
 
 		skb_pull(skb, sizeof(struct mt7615_mcu_rxd));
-		if (data + skb->len - dump > MT76_CONNAC_COREDUMP_SZ) {
+		if (!dump || data + skb->len - dump > MT76_CONNAC_COREDUMP_SZ) {
 			dev_kfree_skb(skb);
 			continue;
 		}
@@ -2390,6 +2390,8 @@ void mt7615_coredump_work(struct work_struct *work)
 
 		dev_kfree_skb(skb);
 	}
+
+	if (dump)
 		dev_coredumpv(dev->mt76.dev, dump, MT76_CONNAC_COREDUMP_SZ,
 			      GFP_KERNEL);
 }