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 <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/f_rndis.c

@@ -374,6 +374,9 @@ static struct sk_buff *rndis_add_header(struct gether *port,
 {
 	struct sk_buff *skb2;
 
+	if (!skb)
+		return NULL;
+
 	skb2 = skb_realloc_headroom(skb, sizeof(struct rndis_packet_msg_type));
 	rndis_add_hdr(skb2);