RFR: JDK-8077265 Modify assert to help debug JDK-8068448

[Eric Caspole]

HI everybody,
Here is a webrev to add more asserts related to debugging JDK-8068448. 
Beyond capturing more info in the original assert, after looking at 
another core I added more asserts to make sure there is no other place 
where old gen allocations would overrun the start array.

  http://cr.openjdk.java.net/~ecaspole/JDK-8077265/00/webrev/

Passes JPRT.
Thanks,
Eric