RFR (S) 8222838: Shenandoah: SEGV on accessing cset bitmap for NULL ptr

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8222838

Webrev:
  http://cr.openjdk.java.net/~shade/8222838/webrev.01/

See the bug for the example failure that this fix resolves. Code comments should tell about the
implementation details for the fix.

Testing: Linux {x86_64, aarch64} hotspot_gc_shenandoah {+Coops, -Coops}, Windows x86_64 ad-hoc
tests, eyeballing NMT summary

-- 
Thanks,
-Aleksey