gui/event-editor/date-time-chooser: Exit if 12-hour time entry is invalid

If the user has 12h time and sets a properly formatted but invalid time,
Calendar would crash because `date_time` would be `NULL`. This would
specifically happen if the time was set over 23:59 AM or 11:59 PM (ex:
19:00 PM).

By breaking if `date_time` is `NULL`, the crash can be avoided.

Closes #1602

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/848>

src/gui/event-editor/gcal-date-time-chooser.c

@@ -287,6 +287,9 @@ validate_and_normalize_time_entry (GcalDateTimeChooser *self)
                                      g_date_time_get_day_of_month (self->date_time),
                                      hour, minute, 0);
 
+        if (!date_time)
+          break;
+
         gcal_date_time_chooser_set_date_time (self, date_time);
       }
       break;