Error in person.get_citation_child_list. Causes error in merging citation in person event reference attribute. (Discovered during investigation of 0006336: Delayed update of bottom bar in Person View after merge)

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/branches/maintenance/gramps34@21195 4ae1f11a-8b86-4847-b8af-ab372f36d1fd