mt7601u: check return value of alloc_skb

Function alloc_skb() will return a NULL pointer if there is no enough
memory. However, in function mt7601u_mcu_msg_alloc(), its return value
is not validated before it is used. This patch fixes it.
Signed-off-by: Pan Bian <bianpan2016@163.com>
Acked-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/mediatek/mt7601u/mcu.c

@@ -66,8 +66,10 @@ mt7601u_mcu_msg_alloc(struct mt7601u_dev *dev, const void *data, int len)
 	WARN_ON(len % 4); /* if length is not divisible by 4 we need to pad */
 
 	skb = alloc_skb(len + MT_DMA_HDR_LEN + 4, GFP_KERNEL);
+	if (skb) {
 		skb_reserve(skb, MT_DMA_HDR_LEN);
 		memcpy(skb_put(skb, len), data, len);
+	}
 
 	return skb;
 }
@@ -170,6 +172,8 @@ static int mt7601u_mcu_function_select(struct mt7601u_dev *dev,
 	};
 
 	skb = mt7601u_mcu_msg_alloc(dev, &msg, sizeof(msg));
+	if (!skb)
+		return -ENOMEM;
 	return mt7601u_mcu_msg_send(dev, skb, CMD_FUN_SET_OP, func == 5);
 }
 
@@ -205,6 +209,8 @@ mt7601u_mcu_calibrate(struct mt7601u_dev *dev, enum mcu_calibrate cal, u32 val)
 	};
 
 	skb = mt7601u_mcu_msg_alloc(dev, &msg, sizeof(msg));
+	if (!skb)
+		return -ENOMEM;
 	return mt7601u_mcu_msg_send(dev, skb, CMD_CALIBRATION_OP, true);
 }