VLAN: Avoid access to non-existing interfaces

VLAN: Avoid access to non-existing interfaces

Currently, hostapd_get_vlan_id_ifname() is used to determine if a given
vlan is valid *and* to actually determine the interface. This leads to
wpa_set_keys() sometimes setting the key on the wildcard interface name,
which does not make sense.

This patch therefore adds hostapd_vlan_id_valid() and makes
hostapd_get_vlan_id_ifname() not return a wildcard interface.

Signed-hostap: Michael Braun <michael-dev@fami-braun.de>