Planner: even with broken plans, don't divide by 0

This fixes the crash in #623, but not the other oddities.

See #623

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Planner: when adding new waypoint at the end, use previous gas

In commit b0da8c270736 ("When adding a waypoint, use the gasmix of the
_next_ waypoint.") we default to air when adding a waypoint at the end of
the plan by double-clicking. That seems silly since we could instead
continue using the last gas.

I wasn't able to reproduce the "gas list disappears" problem in bug #623,
but I did get a silly air segment added which was equally wrong. Maybe I'm
lucky and this fixes the problem that Henrik sees as well...

See #623

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Planner: don't allow the user to remove the last point

This causes all kinds of assumptions to go wrong - and it makes no sense.
Move the point to where you want it or cancel the plan.

Fixes #623

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