Remove an incorrect assertion from in6p_lookup_mcast_ifp().

The socket may be bound to an IPv4-mapped IPv6 address. However, the
inp address is not relevant to the JOIN_GROUP or LEAVE_GROUP operations.

While here remove an unnecessary check for inp == NULL.

Reported by: syzbot+d01ab3d5e6c1516a393c@syzkaller.appspotmail.com
Reviewed by: hselasky
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D25888