This bug may manifest itself not only with the queries for which
the index-merge access method is chosen. It also may display
itself for queries with DISTINCT.

The bug was in how the Unique::get method used the merge_buffers
function. To compare elements in the the queue employed by
merge_buffers() it must use the buffpek_compare function rather
than the function for binary comparison. 


mysql-test/r/innodb_mysql.result:
  Added a test case for bug #25798.
mysql-test/t/innodb_mysql.test:
  Added a test case for bug #25798.
sql/filesort.cc:
  Fixed bug #25798.
  The function merge_buffers() when called from the Uniques::get method
  must use function buffpek_compare to compare elements in the queue it
  employs. The pointer to buffpek_compare and the info for the function
  that compares sorted records are passed to merge_buffers through certain 
  designated fields of the SORTPARAM structure.
sql/sql_sort.h:
  Fixed bug #25798.
  Added fields to the SORTPARAM structure to be used in the function 
  merge_buffers when called by the Uniques::get method.
sql/uniques.cc:
  Fixed bug 25798.
  The function merge_buffers() when called from the Uniques::get method
  must use function buffpek_compare to compare elements in the queue it
  employes.