bug(fe): panic when aggregation on distinct onresult

[st1page]

reproduce

create table t(a int, b int);
explain select count(*) from (
    select distinct on(a,b) * from t
)

output

dev=> explain select count(*) from ( select distinct on(a) * from t);
                                QUERY PLAN
---------------------------------------------------------------------------
 BatchSimpleAgg { aggs: [sum0(count)] }
 └─BatchExchange { order: [], dist: Single }
   └─BatchSimpleAgg { aggs: [count] }
     └─BatchGroupTopN { order: [], limit: 1, offset: 0, group_key: [t.a] }
       └─BatchExchange { order: [], dist: HashShard(t.a) }
         └─BatchScan { table: t, columns: [a] }
(6 rows)

dev=> explain select count(*) from ( select distinct on(b) * from t);
ERROR:  Panicked when handling the request: index out of bounds: the len is 1 but the index is 1
This is a bug. We would appreciate a bug report at:
  https://github.com/risingwavelabs/risingwave/issues/new?labels=type%2Fbug&template=bug_report.yml
server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
The connection to the server was lost. Attempting reset: Succeeded.
psql (16.3, server 13.14.0)