JvmtiTrace::safe_get_current_thread_name is unsafe in debug builds

The no-arg overload of JvmtiTrace::safe_get_current_thread_name calls the one-arg overload with the result of Thread::current(). In a debug build this will assert if there is no current Thread. It should instead pass the result of Thread::current_or_null(), and let the existing NULL check in the one-arg function handle that case.