Can you rollback a query in state 'committing alter table to storage engine'
I am sorry to have to inform you, but ALTER TABLE cannot be rolled back. In fact, ALTER TABLE triggers an implicit commit.

According to Page 418 Paragraph 3 of the Book



the following commands can and will trigger an implicit commit


ALTER TABLE
BEGIN
CREATE INDEX
DROP DATABASE
DROP INDEX
DROP TABLE
RENAME TABLE
TRUNCATE TABLE
LOCK TABLES
UNLOCK TABLES
SET AUTOCOMMIT = 1
START TRANSACTION


I have mentioned this before


Aug 21, 2015 : Transactional DDL workflow for MySQL
Feb 12, 2014 : row locking within ACID transaction innodb
Mar 15, 2013 : MySQL backup InnoDB


In your case, the ALTER TABLE will either finish or you will have a temp table left hanging around. In either case, it will not be a rollback in a transactional sense.