set type->len to the prefix_len of the index column, if it is a prefix index.
This should prevent bugs similar to Bug #21638 from occurring.

dict_index_copy_types(): Set type->len to prefix_len if prefix_len != 0.

row_build_index_entry(): Set type->len to prefix_len if prefix_len != 0,
also when the column in the tuple is SQL NULL, because the type information
may be used for interpreting other records during btr_page_reorganize().