The alleged drunken driver who fatally struck Nassau Police Officer Patricia Espinosa as she drove to work, killing the mother of a toddler, is expected to plead guilty in connection with the case at his next court appearance Thursday, sources told Newsday. Matthew Smith, 20, of Hauppauge, who has been indicted on 19 charges, including aggravated vehicular homicide and aggravated vehicular assault, previously pleaded not guilty. Details of the plea agreement have not yet been disclosed.