Make sure that the planned dive is selected

With the changes to the selection logic the selected_dive variable didn't
get updated at the end of planning a dive. With an empty dive list that
could cause selected_dive to be -1 which would subsequently cause a
SIGSEGV when trying to edit the newly created dive.

With this commit we use the shared go_to_iter() function and also make
sure that selected_dive is set correctly.

Reported-by: Sergey Starosek <sergey.starosek@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>