Bug#11657 Creation of secondary index fails:

  If TINYBLOB key part length is 255, it is equal
  to field length. For BLOB, unlike for CHAR/VARCHAR,
  we should keep a non-zero key part length, otherwise
  "BLOB/TEXT column used in key specification without a key length"
  error is produced afterwards.

type_blob.result, type_blob.test:
  fixing tests accordinly


sql/sql_table.cc:
  Bug#11657 Creation of secondary index fails
  For TINYBLOB key part length can be equal to field length.
  We should still keep a non-zero key part length,
mysql-test/t/type_blob.test:
  fixing tests accordinly
mysql-test/r/type_blob.result:
  fixing tests accordinly