Don't spew warnings when flint doesn't exist

Summary: Before this diff `arc lint` on non-fb machine issued warnings. Now it doesn't.

Test Plan: `arc lint` is quiet.

Reviewers: yhchiang, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D49071

arcanist_util/cpp_linter/FbcodeCppLinter.php

@@ -88,6 +88,9 @@ class FbcodeCppLinter extends ArcanistLinter {
   }
 
   private function getCppLintOutput($path) {
+     if (!array_key_exists($path, $this->rawLintOutput)) {
+       return array();
+     }
     list($output) = $this->rawLintOutput[$path];
 
     $msgs = array();