usb: gadget: function: f_eem: 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: NPeter Chen <peter.chen@nxp.com>
Signed-off-by: NFelipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/f_eem.c

@@ -341,11 +341,15 @@ static struct sk_buff *eem_wrap(struct gether *port, struct sk_buff *skb)
 {
 	struct sk_buff	*skb2 = NULL;
 	struct usb_ep	*in = port->in_ep;
-	int		padlen = 0;
+	int		headroom, tailroom, padlen = 0;
 	u16		len = skb->len;
 
-	int headroom = skb_headroom(skb);
-	int tailroom = skb_tailroom(skb);
+	if (!skb)
+		return NULL;
+
+	len = skb->len;
+	headroom = skb_headroom(skb);
+	tailroom = skb_tailroom(skb);
 
 	/* When (len + EEM_HLEN + ETH_FCS_LEN) % in->maxpacket) is 0,
 	 * stick two bytes of zero-length EEM packet on the end.