!TABLES->NEXT_NAME_RESOLUTION_TABLE) || !TAB
      
Problem:
The context info of select query gets corrupted when a query
with group_concat having order by is present in an order by
clause of the select query. As a result, server crashes with
an assert.
      
Analysis:
While parsing order by for group_concat, it is presumed that
it is always present before the actual order by for the
select query.
As a result, parser uses select->order_list to populate the
order by items of group_concat and creates a select->gorder_list
to which select->order_list is copied onto. Once this is done,
it empties the select->order_list.
In the case presented in the bugpage, as order by is already
parsed when group_concat's order by is encountered, parser
presumes that it is the second order by in the select query
and creates fake_lex_unit which results in the change of
context info.
      
Solution:
Make group_concat's order by parsing independent of the select