[DESK] Fix screensaver preview after calling its settings (#4921)

Fix a bug when after closing the settings window for a screensaver,
the preview for it wouldn't restart.

- Keep waiting for messages from the settings window while it's open
- Fix leaking two handles to avoid creating zombie processes

CORE-18680

dll/cpl/desk/screensaver.c

@@ -247,10 +247,6 @@ WaitForSettingsDialog(HWND hwndDlg,
                     DispatchMessage(&msg);
                 }
             }
-            else
-            {
-                return FALSE;
-            }
         }
         else if (dwResult == WAIT_OBJECT_0)
         {
@@ -308,6 +304,9 @@ ScreensaverConfig(HWND hwndDlg, PDATA pData)
 
         if (WaitForSettingsDialog(hwndDlg, pi.hProcess))
             SetScreenSaverPreviewBox(hwndDlg, pData);
+
+        CloseHandle(pi.hProcess);
+        CloseHandle(pi.hThread);
     }
 }