There is a check on `min_inters` inside `inters_lines` but adding
these two tests in overlap can spare calls to `copy_shift`. Note that
we do not need to add such tests in the three other cases of nullity
of speed contours because, in these three other cases, if at least one
of `min_inters_speed` or `min_inters_out` is <= 1 then `p_shifted` is
used.