RFR(XXXS): 8000311: G1: ParallelGCThreads==0 broken

[John Cuthbertson]

Hi Everyone,

Can I have a couple of volunteers look over the changes for this fix? 
The webrev can be found at: 
http://cr.openjdk.java.net/~johnc/8000311/webrev.0/

Summary:
While testing the changes for another CR, I ran into this divide by zero 
error. I decided to make a separate CR so that the change can be 
backported to hs24 - which also has the issue.

Testing:
The original test case with and without PGCT=0 and PrintPLAB enabled.

Thanks,

JohnC