Trip editing: zero out potential previous edits

This was silly; if we rely on this to be zero to indicate no change then
we better zero it out when we start editing.

Fixes #805

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

qt-ui/maintab.cpp

@@ -310,6 +310,7 @@ void MainTab::enableEdition(EditMode newEditMode)
 	if (MainWindow::instance() && MainWindow::instance()->dive_list()->selectedTrips().count() == 1) {
 		// we are editing trip location and notes
 		displayMessage(tr("This trip is being edited."));
+		memset(&displayedTrip, 0, sizeof(displayedTrip));
 		currentTrip = current_dive->divetrip;
 		ui.dateEdit->setEnabled(false);
 		editMode = TRIP;