Fix crash when manually adding a dive

When a manually added dive is saved we cancel the plan. In that process
we throw away the cylinders which triggers a redraw of the profile. Which
tries to access the diveplan and its data points that have already been
freed. BOOM.

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

qt-ui/diveplanner.cpp

@@ -1404,6 +1404,7 @@ void DivePlannerPointsModel::cancelPlan()
 		free(stagingDive);
 	setPlanMode(NOTHING);
 	stagingDive = NULL;
+	diveplan.dp = NULL;
 	CylindersModel::instance()->setDive(current_dive);
 	CylindersModel::instance()->update();
 }