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

include/wx/buffer.h

@@ -56,12 +56,8 @@ struct UntypedBufferData
     bool m_owned;
 };
 
-// this has to be defined inside the DLL (and not e.g. as a static variable
-// inside an inline function) as otherwise MSVC gives link errors when the
-// functions are effectively inlined (i.e. in non-debug build)
-//
 // NB: this is defined in string.cpp and not the (non-existent) buffer.cpp
-extern WXDLLIMPEXP_DATA_BASE(UntypedBufferData * const) untypedNullDataPtr;
+WXDLLIMPEXP_BASE UntypedBufferData * GetUntypedNullData();
 
 } // namespace wxPrivate
 
@@ -186,7 +182,7 @@ protected:
     // placeholder for NULL string, to simplify this code
     static Data *GetNullData()
     {
-        return static_cast<Data *>(wxPrivate::untypedNullDataPtr);
+        return static_cast<Data *>(wxPrivate::GetUntypedNullData());
     }
 
     void IncRef()