Fix an issue where "phutil_utf8v()" could fatal when passed an integer

Summary:
See https://discourse.phabricator-community.org/t/search-by-name-in-files-doesnt-support-number/4300.

I can't exactly reproduce the original issue, but when a query like "quack 1234" is tokenized, we end up calling "phutil_utf8v(1234)", where the argument is an integer.

At least in recent versions of PHP, this fatals ("trying to access an offset of an integer"). Cast the argument first.

Test Plan: Searched for "quack 1234" in Files. Before: fatal accessing offset of integer; after: correct results.

Differential Revision: https://secure.phabricator.com/D21477