Don't crash when restoring the selection

If we end up trying to restore the selection where the selected dive is no
longer visible (i.e., it's now filtered away), this code caused a crash by
falling first() on an empty list. Let's not do that.

Fixes #758
Be more careful when restoring a selection in the presence of filters

Try really hard to ensure there remains a valid selection. Only if none of
the dives are selectable should we give up.

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