MFC: r303685

truss: fix uninitialized trussinfo->curthread in add_threads()/enter_syscall

trussinfo->curthread must be initialized before calling enter_syscall(),
it is used by t->proc->abi->fetch_args().
Without that truss is segfaulting and the attached program also crash.

Submitted by: Nikita Kozlov (nikita@gandi.net)
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D7399