Start cleaning up dive accessors

Start cleaning up dive accessors

I'm going to add a menu to import (and eventually export) dives, and so
we'd like to be able to start out with no dives at all.  Right now we
croak if that happens - it's not like the code has been written with
actual end users in mind.

So start cleaning things up.  First make the 'current_dive' macro work
right even for invalid dives.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>