Don't call wxTextMeasure::DoGetTextExtent() with NULL width pointer, it now
supposes that both width and height pointers are non-NULL.
Add at least a trivial unit test for GetMultiLineTextExtent().
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72700 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775