Fix mw.ustring edge cases

A few edge cases were being incorrectly handled:
* mw.ustring.sub( 'abc', 1, 0 ) returned 'a', not ''.
* mw.ustring.codepoint( 'abc', 1, 0 ) returned 97, not no results.
* mw.ustring.codepoint( 'abc', 4, 4 ) returned 99, not no results.
* mw.ustring.gcodepoint had the same issues as mw.ustring.codepoint.

Change-Id: Ib8c0ef5a8073106eb7d90d0aa0513be4525dca08
Merge "Fix mw.ustring edge cases"