Avoid crash on abnormal exit in wx+MFC combined applications

Reset m_pMainWnd in wxMFCApp::ExitInstance() to avoid crash when
deleting it again in OnMainWindowDestroyed() that could happen if
ExitInstance() was called not because the main window was closed (normal
case) but because wxApp::ExitMainLoop() was called, as it happens when
an unhandled exception is thrown.

include/wx/msw/mfc.h

@@ -81,6 +81,7 @@ public:
     int ExitInstance() wxOVERRIDE
     {
         delete m_pMainWnd;
+        m_pMainWnd = NULL;
 
         if ( wxTheApp )
             wxTheApp->OnExit();