Check COMMENT_NODE_OPEN on issue edit pages

From formatC'vt in #2552683-5: Issues that are closed for comments shouldn't allow for new ones via issue edit:

"make sure that a locked issue cannot be edited either" should be taken not by nodechanges module i think

/**
 * Implements hook_node_access().
 */
function drupalorg_node_access($node, $op, $account) {
  // If $node is a string, the node has not yet been created. We don't care
  // about that case.
  if (is_string($node)) {
    return NODE_ACCESS_IGNORE;
  }
  if ($op == 'update') {
    if ($node->comment == COMMENT_NODE_OPEN || user_access('administer nodes', $account)) {
      return NODE_ACCESS_ALLOW;
    }
    else {
      return NODE_ACCESS_DENY;
    }
  }
  return NODE_ACCESS_IGNORE;
}

I think this is best in project_issue module, with a check for project_issue_node_is_issue().