Initialize wxXmlResourceHandler members in its ctor.

Initialize wxXmlResourceHandler members in its ctor.

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