oxcical: better error string for event instance group without RRULE

In a VEVENT group (one with the same UID) for a recurrence, there
must be one main event with an RRULE line, and the subordinates with
RECURRENCE-ID lines. If there is no VEVENT with RRULE, the calendar
file is broken and eml2mt rejects it. The messsage was just the
generic:

```
$ eml2mt --ical t.ics >/dev/zero
t.ics: Import rejected for an unspecified reason (usually a too-strict parser).
```

Now this case outputs:

```
E-2739: Some VEVENT.RECURRENCE-ID points to a UID but there was no
RRULE line anywhere
```

References: GXL-598, DESK-3426

Author: jengelh

lib/mapi/oxcical.cpp

@@ -1955,7 +1955,9 @@ static bool oxcical_parse_valarm(uint32_t reminder_delta, time_t start_time,
 static const ical_component *oxcical_main_event(const event_list_t &evlist, const char **err)
 {
 	*err = nullptr;
-	if (evlist.size() == 1)
+	if (evlist.size() == 0)
+		return nullptr;
+	else if (evlist.size() == 1)
 		return evlist.front();
 	const ical_component *main_event = nullptr;
 	for (const auto &event : evlist) {
@@ -1979,6 +1981,8 @@ static const ical_component *oxcical_main_event(const event_list_t &evlist, cons
 			return nullptr;
 		}
 	}
+	if (main_event == nullptr)
+		*err = "E-2739: Some VEVENT.RECURRENCE-ID points to a UID but there was no RRULE line anywhere";
 	return main_event;
 }