RFR: 8238592: JFR: Crash when dumping paths to gc roots on deep heaps

[Erik Gahlin]

Hi,

Could I have review of this fix.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8238592 <https://bugs.openjdk.java.net/browse/JDK-8238592>

Webrev:
http://cr.openjdk.java.net/~egahlin/8238592/ <http://cr.openjdk.java.net/~egahlin/8238592/>

Testing:

jdk/jdk/jfr/*
jdk/jdk/jfr/event/oldobject/* 100 times
jdk/jdk/jfr/event/oldobject/TestLargeRootSet.java on different GCs.

Thanks
Erik