Fix overly large values/nulls arrays

Fix overly large values/nulls arrays

These arrays were sized with Natts_pg_trigger (19) when they should have
been sized with Natts_pg_event_trigger (7).  We'd better fix this as
it's clearly a mistake and it could become problematic if
pg_event_trigger were to gain a dozen or so more columns in the future.

No backpatch as there's no actual bug and the column count on those
tables isn't going to change in released versions.

Author: Xin Zhang 
Discussion: https://postgr.es/m/[email protected]