COPY ALTER is internally implemented as follows:
 a) Create a new table with updated definitions with a temporary name.
 b) Copy rows from the old table to this new table.
 c) Rename old table to a temporary name.
 d) Rename new table to original name.
 e) Drop the old table (now with the temporary name).

If the transaction fails after step (d), the mysqld crashes when it
attempts to rollback all the changes previously made. The crash was
caused due to an out of memory access of a char pointer. Fixed that
issue and added a testcase.

Change-Id: I43ec13592d480baafee5ed113dc4f8dab75291a1