RFR: 8251835: 8251374 breaks jmap -dump:all

[Stefan Karlsson]

Hi all,

Please review this patch to fix a recently introduced jmap bug.

https://cr.openjdk.java.net/~stefank/8251835/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8251835

I added the same kind of checks that we have in histo.

Testing:
- Tested locally with the failing test
- Tier1-tier5 on Linux x64

Paul posted a slightly more elaborate fix that makes dump more akin to 
histo:
http://cr.openjdk.java.net/~phh/8251835/webrev.00/

I don't know the testing status of that patch. If this needs to be fixed 
ASAP, I propose my fix, and then add the rest of Pauls bits as a 
follow-up RFE. If we have time to run Paul's patch through testing, then 
I'm fine with that as well.

Thanks,
StefanK