Optionally return length from wxLoadUserResource().

Optionally return length from wxLoadUserResource().

Add optional length output parameter and also change the return type to "char
*" from "wxChar *" to which it apparently was blindly changed just to make
this code compile even though this function never returned any strings.

Closes #11214.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61977 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775