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
Sponsored by: The FreeBSD Foundation

(cherry picked from commit e755e2776ddff729ae4102f3273473aa33b00077)