ngatm: Handle errors from uni_msg_extend()

uni_msg_extend() may fail due to a memory allocation failure. In this
case, though, the message is freed, so callers shouldn't touch it.

PR: 255861
Reviewed by: harti
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30611