Problem was for LINEAR HASH/KEY. Crashes because of wrong partition id
returned when creating the new altered partitions. (because of wrong
linear hash mask)

Solution: Update the linear hash mask before using it for the new
altered table.


mysql-test/r/partition_hash.result:
  Bug#30822: ALTER TABLE COALESCE PARTITION causes segmentation fault
  
  test result
mysql-test/t/partition_hash.test:
  Bug#30822: ALTER TABLE COALESCE PARTITION causes segmentatition fault
  
  test case
sql/ha_partition.cc:
  Bug#30822: ALTER TABLE COALESCE PARTITION causes segmentation fault
  
  Updating the linear hash mask before using it.
sql/sql_partition.cc:
  Bug#30822: ALTER TABLE COALESCE PARTITION causes segmentation fault
  
  exporting the set_linear_hash_mask function (static -> non static)
sql/sql_partition.h:
  Bug#30822: ALTER TABLE COALESCE PARTITION causes segmentation fault
  
  exporting the set_linear_hash_mask function (static -> non static)