Initialize SLRU stats entries to zero.

Previously since SLRUStats was not initialized, SLRU stats counters
could begin with non-zero value. Which could lead to incorrect results
in pg_stat_slru view.

Author: Fujii Masao
Reviewed-by: Tomas Vondra
Discussion: https://postgr.es/m/976bbb73-a112-de3c-c488-b34b64609793@oss.nttdata.com

src/backend/postmaster/pgstat.c

@@ -2900,6 +2900,9 @@ pgstat_initialize(void)
 		MyBEEntry = &BackendStatusArray[MaxBackends + MyAuxProcType];
 	}
 
+	/* Initialize SLRU statistics to zero */
+	memset(&SLRUStats, 0, sizeof(SLRUStats));
+
 	/* Set up a process-exit hook to clean up */
 	on_shmem_exit(pgstat_beshutdown_hook, 0);
 }