agg_select_candidate, which could cause them to keep more candidates
than they should and thus fail to select a single match.  I had
previously fixed the identical bug in oper_select_candidate, but
didn't realize that the same error was repeated over here.
Also, repair func_select_candidate's curious notion that it could
scribble on the input type-OID vector.  That was causing failure to
apply necessary type coercion later on, leading to malfunction of
examples such as select date('now').