When selection gets confused, don't try to select non-existing dive

While the argument could be made that this is just a symptom of Subsurface
getting very confused about the selection (which it still gets at times -
most likely we are calling select_dive() instead of selectDive()  (or the
corresponding deselect functions) in places where we shouldn't), but
either way, we should not crash.

Fixes #220

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