Fix incorrect field count in pg_control_checkpoint()

18 columns are generated in this function, but we had enough space for
19 of them.  Introduced by 4b0d28d.

Author: Bharath Rupireddy
Reviewed-by: Justin Pryzby, Euler Taveira
Discussion: https://postgr.es/m/CALj2ACVQ=hAs=sT0n4xriimqRrrgECySfg_tSqA+26Rb_yfs2A@mail.gmail.com

src/backend/utils/misc/pg_controldata.c

@@ -79,8 +79,8 @@ pg_control_system(PG_FUNCTION_ARGS)
 Datum
 pg_control_checkpoint(PG_FUNCTION_ARGS)
 {
-	Datum		values[19];
-	bool		nulls[19];
+	Datum		values[18];
+	bool		nulls[18];
 	TupleDesc	tupdesc;
 	HeapTuple	htup;
 	ControlFileData *ControlFile;