Minor fix for JSONLint

Summary: In D10435, the [[https://github.com/Seldaek/jsonlint/ | 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