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

[markj]

Summary

After releasing a 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
MFC after: 3 days