Mutex-lock DB to avoid concurrent access while doing backup DB, as it will be actually locked the DB, and what make Clementine stop if we try to modify it (e.g. add a new playlist)

src/core/database.cpp

@@ -719,6 +719,7 @@ void Database::DoBackup() {
   QSqlDatabase db(this->Connect());
 
   // Before we overwrite anything, make sure the database is not corrupt
+  QMutexLocker l(&mutex_);
   const bool ok = IntegrityCheck(db);
 
   if (ok) {