[RTNETLINK]: Fix netdevice name corruption

When changing a device by ifindex without including a IFLA_IFNAME
attribute, the ifname variable contains random garbage and is used
to change the device name.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -394,6 +394,8 @@ static int rtnl_setlink(struct sk_buff *skb, struct nlmsghdr *nlh, void *arg)
 
 	if (tb[IFLA_IFNAME])
 		nla_strlcpy(ifname, tb[IFLA_IFNAME], IFNAMSIZ);
+	else
+		ifname[0] = '\0';
 
 	err = -EINVAL;
 	ifm = nlmsg_data(nlh);