Avoid potential crash if no dive is selected

I missed to spots where we would unconditionally dereference the dive
pointer.

Reported-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Reported-by: Amit Chaudhuri <amit.k.chaudhuri@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>