RFR (trivial): 8241043: Expand assertions to identify thread with errant _stack_base

[David Holmes]

webrev: http://cr.openjdk.java.net/~dholmes/8241043/webrev/
bug: https://bugs.openjdk.java.net/browse/JDK-8241043

This is a subtask of JDK-8239895 as I've been unable to reproduce the 
failures seen there. So this subtask augments a couple of assertions to 
try and assist with pinning down the actual bug.

I think this counts as trivial.

Testing: tiers 1-3

Thanks,
David