Don't dereference NULL pointer when adding tanks to a dive plan

The code was obviously bogus before.

Fixes #429

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Disable the switching depth column for cylinders

Since the planner is disabled this column isn't needed.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>