WITH SORT ABORTED LEAKS FILE DESCRIPTORS

ISSUE : IO_CACHE used for index_merge quick select
is freed only on successful retrieval of all rows
from index merge.
Suppose if there is a interrupt( or failure) to
this operation of row retrieval (let it be a
KILL_QUERY signal) then we are not freeing the IO_CACHE
resources allocated by index_merge quick select.
And hence temp file associated with it is also not closed.
This lead to a file descriptor leak.

SOLUTION : As part of file sort operation now we always 
free the IO_CACHE allocated by index_merge quick select.