Fix a fatal error in ArcanistXHPASTLinter

Summary: I had assumed that we would always be interested in n_STATEMENT_LIST, which is fine for if (...) { some_statements(); } but not for if (...) one_statement();.

Test Plan: Added a test case.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10551