resolved: exit function if varlink_dispatch() returns > 0

varlink_dispatch() returns > 0 if it already replied to the method call,
hence this is reason to return from the handler function, and not
proceed.

src/resolve/resolved-varlink.c

@@ -1016,7 +1016,7 @@ static int vl_method_resolve_service(Varlink* link, JsonVariant* parameters, Var
                 return -EINVAL;
 
         r = varlink_dispatch(link, parameters, dispatch_table, &p);
-        if (r < 0)
+        if (r != 0)
                 return r;
 
         if (p.ifindex < 0)