RFR: JDK-8222125: Shenandoah: Crash when running with ShenandoahParallelSafepointThreads=1

[Roman Kennke]

Apparently, we're unconditionally calling:

 _safepoint_workers->threads_do(tcl);

however, _safepoint_workers may legally be NULL, when
ShenandoahParallelSafepointThreasds=1. 

Bug:
https://bugs.openjdk.java.net/browse/JDK-8222125
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8222125/webrev.00/

Testing: hotspot_gc_shenandoah

Ok?
Roman
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