socket: Fix a use-after-free in soclose()

After releasing the fd reference to a socket "so", we should avoid
testing SOLISTENING(so) since the socket may have been freed. Instead,
directly test whether the list of unaccepted sockets is empty.

Fixes: f4bb1869ddd2 ("Consistently use the SOLISTENING() macro")
Pointy hat: markj
Sponsored by: The FreeBSD Foundation

(cherry picked from commit dfcef8771484271f2bccdb1dbc088a838441c5a7)