Simplistic solution to add dive number to dive added

If this is the last dive and if the previous dive had a number, give this
dive the next number.

Of course there are a couple of issues with the current implementation. We
have no way of changing the date / time of the added dive, so we KNOW that
it will be the last one. But I put that check there so that once we add
the ability to change the date / time (and hopefully remember to then move
the dive to the correct spot in the dive_table), we only assign a number
if it is the last dive.

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