netinet6: Handle unspecified foreign addresses in in6_pcbconnect()

Prior to commit 90ea8e89d9b751e8b5ae90ef3397883b035788e5, this was
handled by calling in6_pcbladdr().

Reported by: syzkaller
Reviewed by: pouria, glebius
Fixes: 90ea8e89d9b7 ("netinet6: refactor in6_pcbconnect()")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58518