Planner bug

[Sergey Starosek]

Steps to reproduce:

1. Start with empty datafile
2. Create one planned dive which starts in 120 minutes, save it
3. Start creating another planned dive which starts in 60 minutes
(must be earlier then previous one)
4. While editting first segment and navigating out of 'Gas used' field
the first dive disappears from the list

Console shows:

(subsurface:20542): Gtk-CRITICAL **: IA__gtk_tree_model_get_path:
assertion `iter != NULL' failed

(subsurface:20542): Gtk-CRITICAL **: IA__gtk_tree_view_expand_to_path:
assertion `path != NULL' failed

(subsurface:20542): Gtk-CRITICAL **: IA__gtk_tree_view_scroll_to_cell:
assertion `path != NULL || column != NULL' failed

(subsurface:20542): Gtk-CRITICAL **:
IA__gtk_tree_selection_select_iter: assertion `iter != NULL' failed

(subsurface:20542): Gtk-CRITICAL **: IA__gtk_tree_model_get_path:
assertion `iter != NULL' failed

(subsurface:20542): Gtk-CRITICAL **:
IA__gtk_tree_view_set_cursor_on_cell: assertion `path != NULL' failed

Sergey