Don't check recorded requests for unspecified prefix.

We'd check recorded requests for a null prefix, which caused a crash.

message.c

@@ -636,6 +636,7 @@ send_update(struct network *net, int urgent,
     int i;
     struct request *request;
 
+    if(prefix) {
         /* This is needed here, since really_send_update only handles the
            case where network is not null. */
         request = find_request(prefix, plen, NULL);
@@ -648,6 +649,7 @@ send_update(struct network *net, int urgent,
                                 hash_id(route->src->address), net);
             }
         }
+    }
 
     if(net == NULL) {
         for(i = 0; i < numnets; i++)