Prevent renaming interfaces to empty string.

Prevent renaming interfaces to empty string.

Andreas Henriksson wrote:
> From: Alexander Wirt <formorer@debian.org>
>
>
> Signed-off-by: Andreas Henriksson <andreas@fatal.se>
> ---
>  ip/iplink.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
>
> diff --git a/ip/iplink.c b/ip/iplink.c
> index 4060845..da1f64e 100644
> --- a/ip/iplink.c
> +++ b/ip/iplink.c
> @@ -670,6 +670,10 @@ static int do_set(int argc, char **argv)
>  	}
>
>  	if (newname && strcmp(dev, newname)) {
> +		if (strlen(newname) == 0) {
> +		    printf("\"\" is not valid device identifier\n");
> +		    return -1;
> +		}

Indentation fixed, same change for the non-ioctl case, use invarg.
While I'm at it I also fixed the error message for "name too long",
*argv is NULL at this point.

Signed-off-by: Patrick McHardy <kaber@trash.net>