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

[Roman Kennke]

Looks good. Thanks!

Roman

> 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
>