In the "cpplint" binding, raise messages on "line 0" without a line

Summary:
Ref T13543. The "cpplint.py" script may emit messages on line 0, but Arcanist doesn't accept these messages.

This is a small piece of a whole set of broader issues, but stop the bleeding for now.

Test Plan:

• Ran arc lint example.h on a file with no #ifndef guard, and cpplint configured.
• Cpplint raised a message at line "0".
• Before change: arc choked when trying to render this.
• After change: arc survives rendering.

Maniphest Tasks: T13543

Differential Revision: https://secure.phabricator.com/D21289