Replace more MemSet calls with struct initialization

This fixes up 10ea0f924a to use the style introduced by 9fd45870c1.

Author: Richard Guo <guofenglinux@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAMbWs490gJf5A=ydqyjh+Z8mVQa_foTGtcmBtHGLra0aOwLWHQ@mail.gmail.com

src/backend/utils/adt/pgstatfuncs.c

@@ -709,15 +709,11 @@ pg_stat_get_backend_subxact(PG_FUNCTION_ARGS)
 {
 #define PG_STAT_GET_SUBXACT_COLS	2
 	TupleDesc	tupdesc;
-	Datum		values[PG_STAT_GET_SUBXACT_COLS];
-	bool		nulls[PG_STAT_GET_SUBXACT_COLS];
+	Datum		values[PG_STAT_GET_SUBXACT_COLS] = {0};
+	bool		nulls[PG_STAT_GET_SUBXACT_COLS] = {0};
 	int32		beid = PG_GETARG_INT32(0);
 	LocalPgBackendStatus *local_beentry;
 
-	/* Initialise values and NULL flags arrays */
-	MemSet(values, 0, sizeof(values));
-	MemSet(nulls, 0, sizeof(nulls));
-
 	/* Initialise attributes information in the tuple descriptor */
 	tupdesc = CreateTemplateTupleDesc(PG_STAT_GET_SUBXACT_COLS);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "subxact_count",