Protect against overflow when computing metrics.

Reported by Gabriel Kerneis.

message.c

@@ -974,7 +974,7 @@ flushupdates(struct network *net)
                     continue;
                 if(!route_interferes(route, net)) {
                     metric =
-                        route->refmetric +
+                        (int)route->refmetric +
                         (diversity_factor * route->cost / 128) / 256 +
                         route->add_metric;
                     metric = MAX(metric, route->refmetric + 1);

route.h

@@ -44,7 +44,7 @@ struct route {
 static inline int
 route_metric(const struct route *route)
 {
-    int m = route->refmetric + route->cost + route->add_metric;
+    int m = (int)route->refmetric + route->cost + route->add_metric;
     return MIN(m, INFINITY);
 }