Check the result of ToWChar() call in wxTextInputStream::NextChar() better.

The logic in this function depends on ToWChar() working correctly so check
that it doesn't return obviously wrong results, e.g. 0 output length for
non-empty input. This was mostly done to detect the problem in wxConvAuto
currently but it could also be useful with user-defined conversions and
shouldn't have a big performance effect on wxTextInputStream so leave these
checks in to facilitate debugging in the future.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63244 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775