connection: Fix sendmsg() on FreeBSD

It expects ((msg_controllen == 0) == (msg_control == NULL)), and returns
EINVAL otherwise. It can't hurt to be tidy about things on other platforms
either though.

See: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=99356#c5



Signed-off-by: Philip Withnall <philip at tecnocode.co.uk>
Signed-off-by: Karsten Otto <ottoka at posteo.de>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>