Make it possible to use wxCharBuffer during program initialization.

Make it possible to use wxCharBuffer during program initialization.

wxCharBuffer might be used during static initialization, even if only
implicitly. E.g. it is used by wxString::Format() which can be used to
initialize a global string. But it uses the global s_untypedNullData variable
might not be initialized yet resulting in mysterious failures.

Fix this in the usual way by wrapping access to the variable via a function.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63585 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775