Make memset() use sizeof() rather than re-compute size

This is simpler and more closely follows overwhelming precedent.

Report and patch by Mark Dilger.
Discussion: https://postgr.es/m/9A68FB88-5F45-4848-9926-8586E2D777D1@gmail.com

src/backend/statistics/extended_stats.c

@@ -301,9 +301,9 @@ statext_store(Relation pg_stext, Oid statOid,
 	bool		nulls[Natts_pg_statistic_ext];
 	bool		replaces[Natts_pg_statistic_ext];
 
-	memset(nulls, 1, Natts_pg_statistic_ext * sizeof(bool));
+	memset(nulls, true, sizeof(nulls));
-	memset(replaces, 0, Natts_pg_statistic_ext * sizeof(bool));
+	memset(replaces, false, sizeof(replaces));
-	memset(values, 0, Natts_pg_statistic_ext * sizeof(Datum));
+	memset(values, 0, sizeof(values));
 
 	/*
 	 * Construct a new pg_statistic_ext tuple, replacing the calculated stats.