comment_access() doesn't validate the $comment object

Similar function (node_access()) does verify the node object:

  if (!$node || !in_array($op, array('view', 'update', 'delete', 'create'), TRUE)) {
    // If there was no node to check against, or the $op was not one of the
    // supported ones, we return access denied.
    return FALSE;
  }

We can probably do the same and check if the comment is not empty.

  if (!$comment) {
    return FALSE;
  }

I think this would be sufficient, as node module does not verify if the nid is numeric either. It would be great to add a simple test for this along with the simplification of the patch mentioned earlier. Thanks!