nl80211: Check if_indextoname() return value for bridge events

nl80211: Check if_indextoname() return value for bridge events

It would be at least theoretically possible for the bridge netdev to
have disappeared at the time hostapd processes the RTM newlink/dellink
message. As such, it is better to verify that if_indextoname() actually
returned success before printing the bridge ifname in debug. In
addition, there is not much point trying to add the bridge ifindex into
the list of own ifindexes in case the interface has already been
removed, so skip that part as well.

Signed-off-by: Jouni Malinen <j@w1.fi>