6608862: segv in JvmtiEnvBase::check_for_periodic_clean_up()

Reviewed-by: dholmes, dcubed, jcoomes

src/share/vm/runtime/thread.cpp

@@ -2756,7 +2756,13 @@ void Threads::threads_do(ThreadClosure* tc) {
   // For now, just manually iterate through them.
   tc->do_thread(VMThread::vm_thread());
   Universe::heap()->gc_threads_do(tc);
-  tc->do_thread(WatcherThread::watcher_thread());
+  {
+    // Grab the Terminator_lock to prevent watcher_thread from being terminated.
+    MutexLockerEx mu(Terminator_lock, Mutex::_no_safepoint_check_flag);
+    WatcherThread *wt = WatcherThread::watcher_thread();
+    if (wt != NULL)
+      tc->do_thread(wt);
+  }
   // If CompilerThreads ever become non-JavaThreads, add them here
 }