Dive list: use helper function when restoring selection

Looping over the individual dives will be very slow if there are many of
them, as the profile will try to render each of them in succession.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Planner: prevent crash when canceling plan with existing dive list

There is still something seriously wrong here.
For now let's just not restore the selection and at least this prevents us
from crashing. But I still don't understand why it crashes.

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