7006810: G1: Introduce peace-of-mind checking in the Suspendible Thread Set

Add asserts to ensure thread is in the right state.

Reviewed-by: tschatzl, pliden

hotspot/src/share/vm/runtime/thread.cpp

@@ -190,6 +190,7 @@ Thread::Thread() {
   set_stack_size(0);
   set_self_raw_id(0);
   set_lgrp_id(-1);
+  DEBUG_ONLY(clear_suspendible_thread();)
 
   // allocated data structures
   set_osthread(NULL);