8007476: assert(the_owner != NULL) failed: Did not find owning Java thread for lock word address

Make deadlock detection a little more robust in the case of being unable to find the JavaThread associated with an object lock.

Reviewed-by: sla, acorn

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

@@ -813,6 +813,7 @@ JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {
   }
 
   if (owner != NULL) {
+    // owning_thread_from_monitor_owner() may also return NULL here
     return Threads::owning_thread_from_monitor_owner(owner, doLock);
   }