* gc.c (gc_mark_ptr): this check was introduced by accidentaly
  (this is why message is "...", crazy simple) for debugging.
  However, this check is useful because if there is T_NONE
  object here, we can't know which object points to it.
  For debugging reason, I remain this checking code and
  set reasonable error message.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66514 b2dd03c8-39d4-4d8f-98ff-823fe69b080e