RFR (Trivial): Fix the leak of _evacuation_tasks

[Aleksey Shipilev]

On 11/01/2018 05:47 PM, Zhengyu Gu wrote:
> Leak _evacuation_tasks in ShenandoahRootEvacuator.
> 
> Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/leak_evac_tasks/webrev.00/

Looks good. Maybe you want to put the delete first thing to make it symmetrical with
ShenandoahRootProcessor::~ShenandoahRootProcessor().

-Aleksey