RFR(XS): Leak mutex in ShenandoahTaskTerminator

[Aleksey Shipilev]

On 07/19/2017 08:13 PM, Zhengyu Gu wrote:
> Another obvious one (my bad!)
> 
> Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/sh_terminator_memleak/webrev.00/

Yup!

-Aleksey