dhtnode: avoid deadlock in shutdown

tools/dhtnode.cpp

@@ -578,15 +578,19 @@ main(int argc, char **argv)
     std::mutex m;
     bool done {false};
 
-    node->shutdown([&]()
+    node->shutdown([&]() {
         {
+            std::lock_guard<std::mutex> lk(m);
             done = true;
+        }
         cv.notify_all();
     });
 
     // wait for shutdown
+    {
         std::unique_lock<std::mutex> lk(m);
         cv.wait(lk, [&](){ return done; });
+    }
 
     node->join();
 #ifdef WIN32_NATIVE