avoid creating prune message unless we're pruning (#487)

libp2p/protocols/pubsub/gossipsub.nim

@@ -812,7 +812,7 @@ proc heartbeat(g: GossipSub) {.async.} =
         let gossipPeers = g.gossipsub.getOrDefault(t)
         # this will be changed by rebalance but does not matter
         totalMeshPeers += meshPeers.len
-        totalGossipPeers += gossipPeers.len
+        totalGossipPeers += g.gossipsub.peers(t)
         var prunes: seq[PubSubPeer]
         for peer in meshPeers:
           if peer.score < 0.0:
@@ -820,6 +820,7 @@ proc heartbeat(g: GossipSub) {.async.} =
             g.pruned(peer, t)
             g.mesh.removePeer(t, peer)
             prunes &= peer
+        if prunes.len > 0:
           let prune = RPCMsg(control: some(ControlMessage(
             prune: @[ControlPrune(
               topicID: t,