row_build(), row_upd_index_replace_new_col_vals_index_pos(), and
row_upd_index_replace_new_col_vals() are safe.

btr_cur_optimistic_update(), btr_cur_pessimistic_update(): Note that
the B-tree page of the clustered index record is latched in mtr.

trx_undo_prev_version_build(): Add const qualifiers to index_rec
and rec.  Note that the page of index_rec is latched in index_mtr.

row_vers_impl_x_locked_off_kernel(), row_vers_old_has_index_entry():
Note that the stack of versions is locked by mtr and thus it is
safe to call row_build().