minor corrections to TranslateFromUnicodeFormat()

minor corrections to TranslateFromUnicodeFormat()

- Don't try to dereference end() iteratorm this will fail (unlike comparing
  pointer with NUL)
- Use prefix rather than postfix increment for iterator
- Use wxUniChar::GetValue() rather than cast to char for consistency with
  other switch statements in the same file

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@59880 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775