Fix SX Editor not being able to change the Repeat type from "Until".

The way this is stored depends upon the SX End Date being invalid, but
xaccSchedXActionSetLastOccurDate() rejected invalid dates, so once a valid
date had been created, it was impossible to change, except to another valid
date.

src/engine/SchedXaction.c

@@ -629,9 +629,11 @@ xaccSchedXactionGetEndDate(const SchedXaction *sx )
 void
 xaccSchedXactionSetEndDate( SchedXaction *sx, const GDate *newEnd )
 {
-    if (newEnd == NULL
-	|| !g_date_valid( newEnd )
-	|| g_date_compare( newEnd, &sx->start_date ) < 0 )
+/* Note that an invalid GDate IS a permissable value: It means that
+ * the SX is to run "forever". See gnc_sxed_save_sx() and
+ * schedXact_editor_populate() in dialog-sx-editor.c.
+ */
+    if (newEnd == NULL || g_date_compare( newEnd, &sx->start_date ) < 0 )
     {
         /* XXX: I reject the bad data - is this the right
          * thing to do <rgmerk>.