[ospf6d] Fix ospf6d crash if range defined twice

ospf6d will crash if the same range is defined twice.
There was no check if the same range had previously been defined,
thereby causing a later assert to fail.

ospf6d/ospf6_area.c

@@ -353,6 +353,12 @@ DEFUN (area_range,
         UNSET_FLAG (range->flag, OSPF6_ROUTE_DO_NOT_ADVERTISE);
     }
 
+  if (range->rnode)
+    {
+      vty_out (vty, "Range already defined: %s%s", argv[-1], VNL);
+      return CMD_WARNING;
+    }
+
   ospf6_route_add (range, oa->range_table);
   return CMD_SUCCESS;
 }