RFR: 8240530: CheckUnhandledOops breaks BacktraceBuilder::set_has_hidden_top_frame

[Stefan Karlsson]

Hi all,

Please review this patch to get rid of a unhandled oop null check in the 
BacktraceBuilder.

https://cr.openjdk.java.net/~stefank/8240530/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8240530

The patch changes the type from oop to void* to prevent NULL oops from 
being overwritten with non-NULL values.

I think an alternative fix would be to change the type of 
_has_hidden_top_frame to bool. Maybe someone familiar with this code 
knows why this was encoded with an oop NULL check instead of a plain bool?

Testing with tier1-3 with CheckUnhandledOops on by default.

StefanK