ng_tty: copy to the correct mbuf

memcpy should copy to the current mbuf, otherwise we're copying to the
first mbuf in the chain over and over.

Reported by: kevans
Reviewed by: glebius
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D52713

(cherry picked from commit e05521121f05a1b1e0d7c7b5fbe79f83fc303cd0)