Don't unset the comment property before calling functionGetSignature()

Migrated issue

Reported by: jeffschuler

I'm getting many copies of this error -- looks like one for every function comment -- when running cron to trigger the API module to parse drupal core code:
ERROR: Undefined property: PGPClass::$comment on line 739 in sites/all/modules/grammar_parser/engine/grammar_parser.object.inc (Notice)

Patch with a simple fix coming.