MXS-2547: Assert that workers aren't stopped

If a worker is stopped, none of the Worker::execute or post_message
methods should be called.

server/core/worker.cc

@@ -718,6 +718,8 @@ size_t Worker::execute_concurrently(Task& task)
 
 bool Worker::post_message(uint32_t msg_id, intptr_t arg1, intptr_t arg2)
 {
+    ss_dassert(state() != Worker::STOPPED);
+
     // NOTE: No logging here, this function must be signal safe.
     MessageQueue::Message message(msg_id, arg1, arg2);