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