RFR(XS) Fixed memory leak in region garbage cache

[Zhengyu Gu]

An obvious memory leak in ShenandoahHeuristics::get_region_garbage_cache()


Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/choose_cset_memleak/webrev.00/

Test:
   hotspot_gc_shenandoah: release


Thanks,

-Zhengyu