fix memory leak in the usage example

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@49493 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/latex/wx/snglinst.tex

@@ -30,6 +30,9 @@ bool MyApp::OnInit()
     {
         wxLogError(_("Another program instance is already running, aborting."));
 
+        delete m_checker; // OnExit() won't be called if we return false
+        m_checker = NULL;
+
         return false;
     }