networkd: avoid NULL pointer dereference in route_add

If no result parameter is provided, do not attempt to write the
found/newly-created route to it. This is presently not an issue as all
callers currently provide a non-NULL result parameter, however we should
do this for symmetry with address_add and future code robustness.

src/network/networkd-route.c

@@ -322,6 +322,7 @@ int route_add(
         } else
                 return r;
 
+        if (ret)
                 *ret = route;
 
         return 0;