Code review request: 7198529 NPG: assert with NMT code in Thread destructor

[Zhengyu Gu]

Thread's stack base address can be NULL, if the thread is never started 
or exited before calling Thread::record_stack_base_and_size().

Webrev: http://cr.openjdk.java.net/~zgu/7198529/webrev.00/
CR: http://monaco.sfbay.sun.com/detail.jsf?cr=7198529 (not available on 
bugs.sun.com)

Thanks,

-Zhengyu