ERROR WHILE DROPPING INDEX IN 5.7

This bug concerns the check of index prefix lengths for TEXT
type columns. These types of columns has a maximum length
measured in bytes (e.g. 255 bytes for TINYTEXT). However the
index prefix length specified in CREATE TABLE and ALTER TABLE
statements is in characters.

This gave separate buggy behavior for CREATE TABLE and ALTER TABLE:

During CREATE TABLE, the index prefix length was only checked
against storage engine maximum sizes, not the maximum size of
the column (Create_field::key_length is still 0 when the
check was attempted). This e.g. meant that you could create a
prefix index of 768 utf8mb4 characters for TINYTEXT columns
even if the real maximum is 63 characters (255/4).
(768 * 4 = 3072 is the InnoDB max index length).

During ALTER TABLE, the check was broken for multibyte character
sets as we multiplied the maximum size of each column with
bytes_per_character one time too many. This e.g meant that at you
could add an index of 252 utf8mb4 characters for TINYTEXT
columns (63 * 4 = 252).

This patch fixes the problem by comparing the index prefix length
in bytes against the max column length in bytes. So for TINYTEXT,
the maximum prefix is now 63 regardless of whether CREATE TABLE or
ALTER TABLE is used to create the index (assuming a 4 byte
character set is used).