It was possible to issue an ALTER TABLE ADD PRIMARY KEY on
an partitioned InnoDB table that failed and crashed the server.

The problem was that it succeeded to create the PK on at least
one partition, and then failed on a subsequent partition, due to
duplicate key violation. Since the partitions that already had added
the PK was not reverted all partitions was not consistent with the
table definition, which caused the crash.

The solution was to add a revert step to ha_partition::add_index()
that dropped the index for the already succeeded partitions, on failure.

mysql-test/r/partition.result:
  updated result
mysql-test/t/partition.test:
  Added test
sql/ha_partition.cc:
  Only allow ADD/DROP flags in pairs, so that they can be reverted on failures.
  If add_index() fails for a partition, revert (drop the index) for the previous
  partitions.
sql/handler.h:
  Added some extra info in a comment.