There are three problems in Item_func_like::check_covering_prefix_keys:

-Missing check for null value after val_str() call, added.

-Missing chech for THD::error after val_str() call, added.

-Calculation of the const part length of the wild pattern.
 It was calculated using prefix_length * wild_str->charset()->mbmaxlen,
 where prefix_length is number of characters. Then it compared with
 KEY_PART_INF::length mesured in bytes. It does not work properly
 for BLOB fields if wild_str->charset()->mbmaxlen is bigger than 1.
 Now prefix_length is passed to TABLE::update_covering_prefix_keys()
 and KEY_PART_INF::length is divided by mbmaxlen of the Field chararcter
 set.