Add Russel O'Connor to AUTHORS

Apparently he was the original author of commit b0d9ce3858 (Oct ’99).
t/re/pat.t: Keep minitest passing
pp_sys.c:S_doform: remove redundant null check

This static function is called from exactly two places that both die
aforehand if cv is null.  Further, the cv parameter is already marked
as non-null.