Unlock SearchWorker on exit

When breaking the forever loop in SearchWorker::run(), unlock the mutex
of the SearchBar, otherwise it will stay as locked on shutdown.

src/search.cpp

@@ -23,6 +23,7 @@ void SearchWorker::run() {
 		bar->search_mutex.lock();
 		stop = false;
 		if (die) {
+			bar->search_mutex.unlock();
 			break;
 		}
 		// always clear results -> empty search == stop search