Clear statistics and equipment when no dive is selected

This all seems very strange forward.

The reason for the check whether the stats_w widget has been populated is
that at the very beginning, when the UI is still being assembled, a first
call to switch_page() happens as the notebook pages are assembled. At that
point the stats_w widget is still empty which tells us that we aren't
ready to display anything.

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