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

src/lint/linter/ArcanistXHPASTLinter.php

@@ -426,7 +426,7 @@ final class ArcanistXHPASTLinter extends ArcanistBaseXHPASTLinter {
           }
 
           $span = $conditional
-            ->getChildOfType(1, 'n_STATEMENT_LIST')
+            ->getChildByIndex(1)
             ->getTokens();
 
           $whitelist[$type][$symbol_name][] = range(

src/lint/linter/__tests__/xhpast/conditional-usage.lint-test

@@ -12,6 +12,9 @@ if (class_exists('CURLFile')) {
   new DateTimeImmutable();
 }
 
+if (function_exists('json_last_error_msg'))
+  json_last_error_msg();
+
 // These shouldn't fatal.
 if (SomeClass::someStaticMethod($param)) {}
 $var = 'some_func';