Discovery: make sure finished is not called twice, even in case of errors

src/libsync/discovery.cpp

@@ -1173,6 +1173,7 @@ int ProcessDirectoryJob::processSubJobs(int nbJobs)
 void ProcessDirectoryJob::dbError()
 {
     _discoveryData->fatalError(tr("Error while reading the database"));
+    _pendingAsyncJobs = -1; // We're finished, we don't want to emit finished again
     emit finished();
 }
 }