thread.c: fix message

* thread.c (thread_shield_get_mutex): fix object to be shown in
  the message, NULL pointer is useless.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51472 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread.c

@@ -4718,7 +4718,7 @@ thread_shield_get_mutex(VALUE self)
 {
     VALUE mutex = GetThreadShieldPtr(self);
     if (!mutex)
-	rb_raise(rb_eThreadError, "destroyed thread shield - %p", (void *)mutex);
+	rb_raise(rb_eThreadError, "destroyed thread shield - %p", (void *)self);
     return mutex;
 }