The new default database engine for altered table was reassigned to
the old one. That's wrong thing by itself, and (as the engine
for a subpartition gets that new value) leads to DBUG_ASSERTION
in mysql_unpack_partition()


mysql-test/r/partition.result:
  Bug #31893 Partitions: crash if subpartitions and engine change.
  
  test result
mysql-test/t/partition.test:
  Bug #31893 Partitions: crash if subpartitions and engine change.
  
  test case
sql/sql_partition.cc:
  Bug #31893 Partitions: crash if subpartitions and engine change.
  
  Don't change part_info->default_engine_type if it's already set