3RD EXECUTION OF PS

Problem:
When order by is by a column number for a group concat function
which has an outer reference, server fails in case of prepared
statements on the third execution

Analysis:
When a group concat function has order by, the fields in order by
are not resolved until execution if the input is a column number.
During execution they get resolved after the temp table gets created.
As a result they will be pointing to temp table fields which are
runtime created objects. This results in dangling pointers leading
to server failure.

Solution:
Reset the pointers for the order by fields to point to the original
arguments after execution as they are invalid.
Done in Item_func_group_concat::cleanup.