Fix deco planning for low oxygen mixes

Not sure about the future of the current planner.c as we have new
approaches now but as this annoyed me for some days now I just decided
to come up with this trivial stuff to make my life easier.

More a cosmetic fix than a patch, but actually it fixes a bug one might
face planing a dive using low oxygen mixes and where stop levels at just
90m, 60m, 30m and nothing in between will not allow subsurface to finish
a deco ceiling which ends before the universe collapses. Allows to plan
20min@130m using a 10/70 now (f.i.).

Signed-off-by: Jan Schubert / Jan.Schubert@GMX.li
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>