Set thread ID to NULL after termination

After a thread has been terminated set its ID to NULL since the handle
is no longer valid. This prevents accidental use of a deleted thread.

Author: c1728p9

rtos/rtos/Thread.cpp

@@ -102,6 +102,7 @@ osStatus Thread::terminate() {
     _mutex.lock();
 
     ret = osThreadTerminate(_tid);
+    _tid = (osThreadId)NULL;
 
     _mutex.unlock();
     return ret;