route/link: prevent segfault in af_request_type()

[TarEnethil]

ac_request_type() in route/link.c will segfault if the function pointer ops->ao_override_rtm has not been initialized for the given af_type parameter.

The bug was found while trying to call rtnl_link_change() for a link with family type AF_INET6. It seems this bug was introduced in 3c427d0.

Fix: check that ao_override_rtm is a valid pointer before calling it.