This problem comes while inserting a duplicate row in merge table 
without key but the child table has a primary key. 
While forming the error message handler tries to locate the key field
which is creating this problem but as there is no key on the merge
table there is a segmentation fault.

mysql-test/r/merge.result:
  Updated results with new test for this bug.
mysql-test/t/merge.test:
  Added new test to test error generated from a key on child table 
  where merge table doesn't have any key.
storage/myisammrg/ha_myisammrg.cc:
  Added a new check to see if the value of error key is higher than 
  the number of key in merge table and if it is the error key set 
  to MAX_KEY. The error message generation routine treats MAX_KEY as 
  unknown key and doesn't tries to access this in key_info.