Fix git cl comments when there's no issue associated with the current branch.

It currently crashes, which is sad.

R=pgervais@chromium.org
BUG=

Review URL: https://codereview.chromium.org/683283006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@292899 0039d316-1c4b-4281-b951-d872f2087c98

git_cl.py

@@ -1462,7 +1462,7 @@ def CMDcomments(parser, args):
     return 0
 
   data = cl.GetIssueProperties()
-  for message in sorted(data['messages'], key=lambda x: x['date']):
+  for message in sorted(data.get('messages', []), key=lambda x: x['date']):
     if message['disapproval']:
       color = Fore.RED
     elif message['approval']: