Don't initialize wxGBSpan with invalid values in wxGBSizerItem.

Default wxGBSizerItem ctor initialized wxGBSpan with invalid values which
resulted in an assert when loading wxGBSizer from XRC since r66964.

Fix this by simply using the default wxGBSpan ctor instead.

Closes #13004.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67127 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/gbsizer.cpp

@@ -76,7 +76,6 @@ wxGBSizerItem::wxGBSizerItem( wxSizer *sizer,
 wxGBSizerItem::wxGBSizerItem()
     : wxSizerItem(),
       m_pos(-1,-1),
-      m_span(-1,-1),
       m_gbsizer(NULL)
 {
 }