Ensure we don't crash in wxProgressDialog::DoGetPosition()

Check that we do have the shared data before dereferencing the pointer
to it. While this normally will always be the case, it could be null if
some error happened, so add a check for it, just as we already do it
elsewhere.

src/msw/progdlg.cpp

@@ -746,6 +746,7 @@ void wxProgressDialog::DoGetPosition(int *x, int *y) const
     if ( HasNativeTaskDialog() )
     {
         wxPoint pos;
+        if ( m_sharedData )
         {
             wxCriticalSectionLocker locker(m_sharedData->m_cs);
             pos = m_sharedData->m_winPosition;