The patch also fixes an unrelated compiler warning.

Analysis:
The temporary table created during SJ-materialization
might be used for sorting for a group by operation. The
sort buffers for this internal temporary table were not
cleared by the execution code after each subquery
re-execution. This resulted in a memory leak detected
by valgrind.

Solution:
Cleanup the sort buffers for the semijon tables as well.


sql/item_subselect.cc:
  - Fix a compiler warning and add logic to revert to table
    scan partial match when there are more rows in the materialized
    subquery than there can be bits in the NULL bitmap index used
    for partial matching.
sql/opt_subselect.cc:
  - Fixed a memory leak detected by valgrind