Not sure if this results in any real problems but at the very least memory
checking tools give errors about reading uninitialized memory in
wxXmlResourceHandlerImpl::CreateResource() if we leave them uninitialized, so
do initialize all the pointers.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74011 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
