BUG #18755: Using Natts_pg_trigger rather than Natts_pg_event_trigger for event trigger

The following bug has been logged on the website:

Bug reference: 18755
Logged by: Xin Zhang
Email address: zhanghien(at)qq(dot)com
PostgreSQL version: 17.2
Operating system: Any
Description:

In function insert_event_trigger_tuple, the datum array is declared with
length Natts_pg_trigger:
```c
Datum values[Natts_pg_trigger];
bool nulls[Natts_pg_trigger];
```

Natts_pg_trigger should only be used in pg_trigger tuple and
Natts_pg_event_trigger is for pg_event_trigger?
I think this could be a typo. Maybe the author of function
insert_event_trigger_tuple copied the variables from CreateTriggerFiringOn
but forgot to change the array length to Natts_pg_event_trigger.

The current code works because Natts_pg_trigger is 19 and
Natts_pg_event_trigger is 7, the pg_event_trigger values array is allocated
with some extra items which is never accessed with index >= 7.