The problem was that doing ALTER TABLE on a table which had a key
on a TEXT/BLOB column with a prefix longer than the maximum number
of characteres in this column (as per the character set), by mistake,
caused an error (Error 1170 - ER_BLOB_KEY_WITHOUT_LENGTH).

This bug not repeatable in 5.5.

This patch adds a regression test to alter_table.test and
contains no code changes.