7198529: NPG: assert with NMT code in Thread destructor

Summary: Thread stack's base address can be NULL if it is not started or exited before recording the base
Reviewed-by: kvn, fparain

src/share/vm/runtime/thread.cpp

@@ -328,8 +328,14 @@ Thread::~Thread() {
   // Reclaim the objectmonitors from the omFreeList of the moribund thread.
   ObjectSynchronizer::omFlush (this) ;
 
-  address low_stack_addr = stack_base() - stack_size();
-  MemTracker::release_thread_stack(low_stack_addr, stack_size(), this);
+  // stack_base can be NULL if the thread is never started or exited before
+  // record_stack_base_and_size called. Although, we would like to ensure
+  // that all started threads do call record_stack_base_and_size(), there is
+  // not proper way to enforce that.
+  if (_stack_base != NULL) {
+    address low_stack_addr = stack_base() - stack_size();
+    MemTracker::release_thread_stack(low_stack_addr, stack_size(), this);
+  }
 
   // deallocate data structures
   delete resource_area();