Don't call neighbour_rxcost() in the middle of a message

neighbour_rxcost() calls update_neighbour which might send some messages,
so make sure we don't call it in the middle of the txcost message.

message.c

@@ -727,6 +727,8 @@ send_txcost(struct neighbour *neigh, struct network *net)
         }
         net->txcost_time = now.tv_sec;
     } else {
+        int rxcost;
+
         if(net && neigh->network != net)
             return;
 
@@ -737,10 +739,11 @@ send_txcost(struct neighbour *neigh, struct network *net)
                format_address(neigh->id),
                format_address(neigh->address));
 
+        rxcost = neighbour_rxcost(neigh);
         start_message(net, 20);
         accumulate_byte(net, 3);
         accumulate_byte(net, 0);
-        accumulate_short(net, neighbour_rxcost(neigh));
+        accumulate_short(net, rxcost);
         accumulate_data(net, neigh->id, 16);
         schedule_flush(net);
     }