fix deleting a parent may lead to a stale object error (for v2.1.6)

Redmine Defect 7920
Attempted to update a stale object when copying a project
http://www.redmine.org/issues/7920

This revison passes Redmine test.
http://www.redmine.org/projects/redmine/repository/entry/trunk/test/unit/issue_nested_set_test.rb?rev=12679#L244

https://travis-ci.org/marutosi/redmine/jobs/17215987#L1979

<pre>
  1) Error:
test_destroy_parent_issue_updated_during_children_destroy(IssueNestedSetTest):
ActiveRecord::StaleObjectError: Attempted to destroy a stale object: Issue
    app/models/issue.rb:179:in `destroy'
    test/unit/issue_nested_set_test.rb:250:in `test_destroy_parent_issue_updated_during_children_destroy'
    test/unit/issue_nested_set_test.rb:249:in `test_destroy_parent_issue_updated_during_children_destroy'
</pre>