124231: field variable dialog: check access to current field as it could be unavailable caused by its change and a followed removal from the document by e.g. a conditional hide of a section

	cherry-picked from trunk



git-svn-id: https://svn.apache.org/repos/asf/openoffice/branches/AOO410@1611555 13f79535-47bb-0310-9956-ffa450edef68