sys_thread_t could go away before SCHED_LOCK is obtained

From looking at the code, it appears that a thread's sys_thread_t
structure can go away from underneath a thread that is trying
to operate on it.

An example scenario is this:

    Thread.interrupt() {
        sysThreadInterrupt(SYSTHREAD(tid));
    }

    sysThreadInterrupt(sys_thread_t *tid) {
        SCHED_LOCK();
        tid->interrupted = 1;
        if (tid->state == CONDVAR_WAIT) {
....
    }

Between the time SYSTHREAD(tid) is called and the SCHED_LOCK()
is grabbed, the tid can exit. This means that the code in
sysThreadInterrupt() will be using an invalid sys_thread_t.

The main problem is that we don't check to see that the sys_thread_t
is still alive after the SCHED_LOCK() is held.

stuart.ritchie@Eng 1996-10-17