[evolution-data-server] Correct e_cal_component_get_start_plus_duration()

[Milan Crha <mcrha src gnome org>]

commit d35cfaf4310aabe569552c7a66e97e701602efef
Author: Milan Crha <mcrha redhat com>
Date:   Thu Jun 13 15:17:55 2019 +0200

    Correct e_cal_component_get_start_plus_duration()
    
    The returned duration can be a _null_duration or a _bad_duration,
    even when the returned ICalDuration object is not NULL.
    
    Reported as part of the https://gitlab.gnome.org/GNOME/evolution/issues/484

 src/calendar/libecal/e-cal-component.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/src/calendar/libecal/e-cal-component.c b/src/calendar/libecal/e-cal-component.c
index 0db31a276..baf5ddaff 100644
--- a/src/calendar/libecal/e-cal-component.c
+++ b/src/calendar/libecal/e-cal-component.c
@@ -1700,8 +1700,10 @@ e_cal_component_get_start_plus_duration (ECalComponent *comp)
        guint dur_days, dur_hours, dur_minutes, dur_seconds;
 
        duration = i_cal_component_get_duration (comp->priv->icalcomp);
-       if (!duration)
+       if (!duration || i_cal_duration_is_null_duration (duration) || i_cal_duration_is_bad_duration 
(duration)) {
+               g_clear_object (&duration);
                return NULL;
+       }
 
        /* Get the DTSTART time. */
        dt = get_datetime (comp->priv->icalcomp, I_CAL_DTSTART_PROPERTY, i_cal_property_get_dtstart);