allow wxDialog::ShowModal() after Show() (see #1416, #4658, #4664)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@54328 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/dialog.cpp

@@ -283,8 +283,7 @@ void wxDialog::Raise()
 // show dialog modally
 int wxDialog::ShowModal()
 {
-    wxASSERT_MSG(!IsShown(),
-                 _T("ShowModal() can't be called for an already shown dialog"));
+    wxASSERT_MSG( !IsModal(), _T("ShowModal() can't be called twice") );
 
     m_endModalCalled = false;