The problem was an ab-use of last_rkey_length.
Formerly we saved the packed key length (of the search key)
in this element. But in certain cases it got replaced by 
the (packed) result key length.
Now we use a new element of MI_INFO to save the packed key 
length of the search key.


myisam/mi_dbug.c:
  Bug#9112 - Merge table with composite index producing invalid results with some queries
  Fixed the recognition of NULL values in _mi_print_key().
myisam/mi_rkey.c:
  Bug#9112 - Merge table with composite index producing invalid results with some queries
  Saved the packed key length in a new element of MI_INFO.
myisam/mi_search.c:
  Bug#9112 - Merge table with composite index producing invalid results with some queries
  Added a comment and trace prints.
myisam/myisamdef.h:
  Bug#9112 - Merge table with composite index producing invalid results with some queries
  Added a new element to store the packed key length
  for use by the MyISAMMRG engine.
myisammrg/myrg_rkey.c:
  Bug#9112 - Merge table with composite index producing invalid results with some queries
  Changed to use the new element of MI_INFO to get at the
  packed key length.
mysql-test/r/merge.result:
  Bug#9112 - Merge table with composite index producing invalid results with some queries
  The test result.
mysql-test/t/merge.test:
  Bug#9112 - Merge table with composite index producing invalid results with some queries
  The test case.