Don't allow T_NIL in gc_is_moveable_obj

gc_is_moveable_obj is only given GC managed objects, and T_NIL cannot be
a GC managed type.

gc.c

@@ -9511,7 +9511,6 @@ gc_is_moveable_obj(rb_objspace_t *objspace, VALUE obj)
 
     switch (BUILTIN_TYPE(obj)) {
       case T_NONE:
-      case T_NIL:
       case T_MOVED:
       case T_ZOMBIE:
         return FALSE;