RFR (S) 8231197: Shenandoah: JVMTI heap walking cleanup crashes with NULL forwardee

[Roman Kennke]

I think it's ok for now.

Long-term, the JVMTI heapwalking should be done by GC as well to ensure 
consistency. I can already spot some subtle bugs there concerning 
concurrent roots cleaning.

Roman


> Bug:
>    https://bugs.openjdk.java.net/browse/JDK-8231197
> 
> Fix:
>    https://cr.openjdk.java.net/~shade/8231197/webrev.02/
> 
> This reliably reproduces with vmTestbase_nsk_jvmti. The reason is described in the comment there:
> weird clash between JVMTI-own walk of heap and Shenandoah. ShenandoahCodeRoots assert also needs
> relaxing, and I relaxed it only for JVMTI heap walk.
> 
> Testing: hotspot_gc_shenandoah, vmTestbase_nsk_jvmti
>