Merge pull request #3441 from peat-psuwit/for-upstream/dbqueue-stop-mutex

db: fix dead lock in zmDbQueue::stop()

src/zm_db.cpp

@@ -238,8 +238,13 @@ zmDbQueue::~zmDbQueue() {
 }
 
 void zmDbQueue::stop() {
+  {
+    std::unique_lock<std::mutex> lock(mMutex);
+
     mTerminate = true;
     mCondition.notify_all();
+  }
+
   if (mThread.joinable()) mThread.join();
 }