row_merge_create_fts_sort_index(): Initialize dict_col_t.

This fixes an access to uninitialized dict_col_t::ind when a debug
assertion in MariaDB 10.4 invokes is_dropped() in
rec_get_converted_size_comp_prefix_low(). Older MariaDB versions
seem to be unaffected by the uninitialized values, but it should
not hurt to initialize everything.