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

[nikita_elyzion.net]

Summary

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 crash also.

It can be reproduced by trying to attach a binary which is doing a
while (1) usleep(1);

Test Plan

Try to attach a few times a program which is doing a while (1) usleep(1) or any program doing a lot of short syscalls.

[jhb]

Hmm, you actually already know the thread (it's 't'). I think this is fallout from considering removing 'curthread' (but something I haven't done yet). I think you can just do 'info->curthread = t;' before enter_syscall() in place of find_thread().

(Long term I think I'd like to pass 't' down to the ABI callbacks and remove curthread completely. trussinfo itself is an odd duck as it's really just global variables but obfuscated)

[nikita_elyzion.net]

updating with jhb comments