This crash occured after ALTER TABLE was used on a temporary
transactional table locked by LOCK TABLES. Any later attempts to
execute LOCK/UNLOCK TABLES, caused the server to crash.

The reason for the crash was the list of locked tables would
end up having a pointer to a free'd table instance. This happened
because ALTER TABLE deleted the table without also removing the
table reference from the locked tables list.

This patch fixes the problem by making sure ALTER TABLE also
removes the table from the locked tables list.

Test case added to innodb_mysql.test.