Processing WM_CLOSE with the right return value

Ref:
https://docs.microsoft.com/en-us/windows/win32/winmsg/wm-close

PowerEditor/src/NppBigSwitch.cpp

@@ -2206,6 +2206,9 @@ LRESULT Notepad_plus::process(HWND hwnd, UINT message, WPARAM wParam, LPARAM lPa
 			if (_isEndingSessionButNotReady) 
 				::DestroyWindow(hwnd);
 
+			if (message == WM_CLOSE)
+				return 0;
+
 			return TRUE;
 		}