add missing null check

add missing null check

I have a unbound forward zone configured on my router for my $DAYJOB.
The address associated with the zone is only accessible when the router
is connected to a VPN. If the VPN connection is absent, trying to
resolve any domain that must be handled by the zone crashes unbound.
Turns out there's a missing NULL check in `comm_point_send_udp_msg()`.
The same routine already has `if (addr) {} else {}` branches so I guess
protecting the call to `log_addr()` using the same conditional is
reasonable

I have also committed the same fix to unbound shipped with OpenBSD[1].

[1] https://marc.info/?l=openbsd-cvs&m=160993335615698&w=2