Fix various issues with the dive add / edit manual dive code

- get_gas_from_events does NOT always set o2/he. It only updates them IFF
  a matching event is found; so we need to make sure we start out with a
  valid gas mix
- the way we tried to restore the edited dive in case of an edit to a
  manually added that is cancelled was completely bogus. Way too complex
  when we can simply and reliably simply store the dive and then copy it
  back

Fixes #270

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

planner.c

@@ -57,6 +57,8 @@ void set_last_stop(bool last_stop_6m)
 
 void get_gas_from_events(struct divecomputer *dc, int time, int *o2, int *he)
 {
+	// we don't modify the values passed in if nothing is found
+	// so don't call with uninitialized o2/he !
 	struct event *event = dc->events;
 	while (event && event->time.seconds <= time) {
 		if (!strcmp(event->name, "gaschange")) {