avoid a bug w/ exit()

proc.c

@@ -126,6 +126,7 @@ copyproc(struct proc *p)
       kfree(np->kstack, KSTACKSIZE);
       np->kstack = 0;
       np->state = UNUSED;
+      np->parent = 0;
       return 0;
     }
     memmove(np->mem, p->mem, np->sz);