dded code to clear the tail of the reference buffer if the actual key length 
is less than the maximum key length.


mysql-test/r/bdb.result:
  bug#2688 - Wrong index_merge query results for BDB table with variable length primary key.
  Added the test case results.
mysql-test/t/bdb.test:
  bug#2688 - Wrong index_merge query results for BDB table with variable length primary key.
  Added the test case.
sql/ha_berkeley.cc:
  bug#2688 - Wrong index_merge query results for BDB table with variable length primary key.
  Added code to clear the tail of the reference buffer if the actual key length 
  is less than the maximum key length.