Fix wxStyledTextCtrl compilation in non-Unicode build.

The value returned by wx2stc() in non-Unicode build is a simple "char*" and we
can't call length() method on it, add a helper wx2stclen() function to get the
length from the original wxString itself in this case.

Closes #14013.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71428 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775