The non documented command 'ALTER PARTITION t REORGANIZE PARTITION'
(without any partitions!) which only make sense for nativly
partitioned engines, such as NDB, crashes the server if there was
no change of number of partitions.

The problem was wrong usage of fast_end_partition function,
which led to usage of a non initialized variable.

mysql-test/r/partition_mgm.result:
  Bug#40389: REORGANIZE PARTITION crashes when only using one partition
  
  Updated test result.
mysql-test/t/partition_mgm.test:
  Bug#40389: REORGANIZE PARTITION crashes when only using one partition
  
  Added new test case.
sql/partition_info.cc:
  Bug#40389: REORGANIZE PARTITION crashes when only using one partition
  
  Added DBUG_ASSERT to easier catch similar problems.
sql/sql_partition.cc:
  Bug#40389: REORGANIZE PARTITION crashes when only using one partition
  
  fast_end_partitions is called later in mysql_alter_table if
  variable fast_alter_partition is set.