Minor fix for JSONLint

Summary: In D10435, the JSONLint external was patched in an attempt to ensure PHP 5.2 compatibility. The fact that $this->lexer->lex() will produce a different token if invoked a second time was overlooked.

Test Plan: Ran ./bin/arc lint -- src/lint/linter in rARC.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10448