procfs ioctl, PIOCTSTOP was not covered to handle the possibility of the debugged process doing an exit. 

stack of the defunct thread.
>  0 thread_block         src/kernel/kern/sched_prim.c : 3303
   1 exit                 src/kernel/bsd/kern_exit.c : 1254
   2 sigexit              src/kernel/bsd/kern_sig.c : 6170
   3 psig                 src/kernel/bsd/kern_sig.c : 5876
   4 syscall              src/kernel/arch/alpha/syscall_trap.c : 842
   5 _Xsyscall            src/kernel/arch/alpha/locore.s : 1882

