Fix: abort GRFFileScanner on exiting the game as soon as possible

This prevents the window from "freezing" when you close it during
the scanning of NewGRFs, as it first would continue the action.

src/newgrf_config.cpp

@@ -603,6 +603,9 @@ public:
 
 bool GRFFileScanner::AddFile(const std::string &filename, size_t basepath_length, const std::string &tar_filename)
 {
+	/* Abort if the user stopped the game during a scan. */
+	if (_exit_game) return false;
+
 	GRFConfig *c = new GRFConfig(filename.c_str() + basepath_length);
 
 	bool added = true;
@@ -702,7 +705,7 @@ void DoScanNewGRFFiles(NewGRFScanCallback *callback)
 	/* Yes... these are the NewGRF windows */
 	InvalidateWindowClassesData(WC_SAVELOAD, 0, true);
 	InvalidateWindowData(WC_GAME_OPTIONS, WN_GAME_OPTIONS_NEWGRF_STATE, GOID_NEWGRF_RESCANNED, true);
-	if (callback != nullptr) callback->OnNewGRFsScanned();
+	if (!_exit_game && callback != nullptr) callback->OnNewGRFsScanned();
 
 	DeleteWindowByClass(WC_MODAL_PROGRESS);
 	SetModalProgress(false);

src/openttd.cpp

@@ -1469,6 +1469,8 @@ void GameLoop()
 		ScanNewGRFFiles(_request_newgrf_scan_callback);
 		_request_newgrf_scan = false;
 		_request_newgrf_scan_callback = nullptr;
+		/* In case someone closed the game during our scan, don't do anything else. */
+		if (_exit_game) return;
 	}
 
 	ProcessAsyncSaveFinish();