usb: gadget: function: f_rndis: socket buffer may be NULL

In eth_start_xmit, the socket buffer may be NULL. So, add NULL pointer
check at .wrap API.

Signed-off-by: Peter Chen <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

drivers/usb/gadget/function/f_rndis.c

···
 {
 	struct sk_buff *skb2;
 
+	if (!skb)
+		return NULL;
+
 	skb2 = skb_realloc_headroom(skb, sizeof(struct rndis_packet_msg_type));
 	rndis_add_hdr(skb2);