A FROM SUBQUERY

Problem:-
Full-text search combined with derived tables causes a segment fault.

Analysis:-
In query like
EXPLAIN SELECT  MATCH(ft) AGAINST( "test" IN BOOLEAN MODE) FROM
( SELECT  ft FROM t1 ) AS t;

We are creating a temporary table for the subquery result table.
But after wl-5274(where we are postponed materialization), we will create
tmp table, but skip instantiation of temporary table.

When we have full-text function in our main select query, we are
going to call Item_func_match::init_search() for all the full-text item.
There we are going to take care about
condition like
--> match ... against (null)
and we are going to call handler::ft_init_ext(), which is used to
Initialize full-text index scan(which need table to be initialized).
but as we havnt initialized the table. This cause a segmentation fault.

Solution:-
When we have derived tables, instead of intializing full-text function at 
optimization, we will initiate it at the time of execution .