unix: Fix handling of listening sockets during garbage collection

socantrcvmore() and unp_dispose() assume that the socket's socket
buffers are initialized, which isn't the case for listening sockets.

Reported by: syzbot+a62883292a5c257703be@syzkaller.appspotmail.com
MFC after: 1 week
Reviewed by: glebius
Differential Revision: https://reviews.freebsd.org/D53743