Fix stats.arenas.<i>.[...] for --disable-stats case.

Add missing stats.arenas.<i>.{dss,lg_dirty_mult,decay_time}
initialization.

Fix stats.arenas.<i>.{pactive,pdirty} to read under the protection of
the arena mutex.

include/jemalloc/internal/ctl.h

@@ -38,6 +38,9 @@ struct ctl_arena_stats_s {
 	ssize_t			decay_time;
 	size_t			pactive;
 	size_t			pdirty;
+
+	/* The remainder are only populated if config_stats is true. */
+
 	arena_stats_t		astats;
 
 	/* Aggregate stats for small size classes, based on bin stats. */