check commit before calling its method

Summary: it's calling pull daemon's failure. This is actually Nick's fix.

Test Plan: Nick already manually ran it on daemon machine.

Reviewers: vrana, nh

Reviewed By: vrana

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2828

src/applications/repository/daemon/PhabricatorRepositoryPullLocalDaemon.php

@@ -264,6 +264,10 @@ final class PhabricatorRepositoryPullLocalDaemon
       $repository->getID(),
       $target);
 
+    if (!$commit) {
+      return false;
+    }
+
     $data = $commit->loadCommitData();
     if (!$data) {
       return false;