mark thread name string

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50869 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

vm.c

@@ -2126,6 +2126,8 @@ rb_thread_mark(void *ptr)
 				 sizeof(th->machine.regs) / sizeof(VALUE));
 	}
 
+	RUBY_MARK_UNLESS_NULL(th->name);
+
 	rb_vm_trace_mark_event_hooks(&th->event_hooks);
     }