xen/netback: fix build warning

Commit ad7f402a ("xen/netback: Ensure protocol headers don't fall in
the non-linear area") introduced a (valid) build warning. There have
even been reports of this problem breaking networking of Xen guests.

Fixes: ad7f402a ("xen/netback: Ensure protocol headers don't fall in the non-linear area")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Tested-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/net/xen-netback/netback.c

@@ -530,7 +530,7 @@ static int xenvif_tx_check_gop(struct xenvif_queue *queue,
 	const bool sharedslot = nr_frags &&
 				frag_get_pending_idx(&shinfo->frags[0]) ==
 				    copy_pending_idx(skb, copy_count(skb) - 1);
-	int i, err;
+	int i, err = 0;
 
 	for (i = 0; i < copy_count(skb); i++) {
 		int newerr;