DB_COL_APPEARS_TWICE_IN_INDEX: Remove. This condition is already
checked and reported by MySQL before passing the index definition to
the storage engine.

row_create_index_for_mysql(): Remove the redundant check for
DB_COL_APPEARS_TWICE_IN_INDEX. When enforcing the column prefix index
limit, invoke dict_mem_index_free(index) to plug the memory leak. In
the loop, use index->n_def instead of dict_index_get_n_fields(index),
because the latter would be 0 for indexes that have not been copied to
the data dictionary cache.

innodb-use-sys-malloc.test:

Add test cases for attempting to trigger the error checks in
row_create_index_for_mysql(). Before MySQL 5.5 and WL#5743, the leak
is only reproducible if ha_innobase::max_supported_key_part_length()
returned a higher limit than the one used in
row_create_index_for_mysql().

In MySQL 5.5 and later, the leak is reproducible with
innodb_large_prefix=true.

rb:688 approved by Jimmy Yang