This assertion would fail when a secondary index record for an
instantly added column was accessed.

It is unclear to me why this code path is executed so rarely.
I was unable to cover it even when using FORCE INDEX.

row_sel_sec_rec_is_for_clust_rec(): Remove the assertion, and use
the proper function rec_get_nth_cfield().

row_sel_store_mysql_field_func(): Simply use rec_get_nth_cfield()
instead of duplicating its logic.