Bug: Issues with write barrier for __newindex

In 'luaV_finishset', there is an update on a table that is a field on
another table. If the first table is the same as the one with the field
(e.g., after 't.__newindex = t'), the update can change the value on
that field (e.g., there may be a collision and the field is moved, or
the field being updated is '__newindex' itself). After that, the
barrier is called with the table stored in that field, which is not
the correct table anymore.