net: rtnl: avoid uninitialized data in IFLA_VF_VLAN_LIST handling

With the newly added support for IFLA_VF_VLAN_LIST netlink messages,
we get a warning about potential uninitialized variable use in
the parsing of the user input when enabling the -Wmaybe-uninitialized
warning:

net/core/rtnetlink.c: In function 'do_setvfinfo':
net/core/rtnetlink.c:1756:9: error: 'ivvl$' may be used uninitialized in this function [-Werror=maybe-uninitialized]

I have not been able to prove whether it is possible to arrive in
this code with an empty IFLA_VF_VLAN_LIST block, but if we do,
then ndo_set_vf_vlan gets called with uninitialized arguments.

This adds an explicit check for an empty list, making it obvious
to the reader and the compiler that this cannot happen.

Fixes: 79aab093 ("net: Update API for VF vlan protocol 802.1ad support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>