Problem was usage of read_range_first with an empty key.

Solution was to not to give a key if it was empty.

mysql-test/r/partition.result:
  Bug#40954: Crash in MyISAM index code with concurrency test using partitioned tables
  
  Updated test result.
mysql-test/t/partition.test:
  Bug#40954: Crash in MyISAM index code with concurrency test using partitioned tables
  
  Added test case