hg: shenandoah/jdk: Handle scenario that not be able to satisfy the request for active workers

[zgu at redhat.com]

Changeset: 2ddeb3c0d9a1
Author:    zgu
Date:      2018-10-04 08:27 -0400
URL:       http://hg.openjdk.java.net/shenandoah/jdk/rev/2ddeb3c0d9a1

Handle scenario that not be able to satisfy the request for active workers

! src/hotspot/share/gc/shenandoah/shenandoahWorkGroup.cpp