rpc: wrap canceling a breach watch in a select case

rpcserver.go

@@ -613,7 +613,11 @@ func (r *rpcServer) CloseChannel(in *lnrpc.CloseChannelRequest,
 			r.server.htlcSwitch.RemoveLink(chanID)
 		}
 
-		r.server.breachArbiter.settledContracts <- chanPoint
+		select {
+		case r.server.breachArbiter.settledContracts <- chanPoint:
+		case <-r.quit:
+			return fmt.Errorf("server shutting down")
+		}
 
 		// With the necessary indexes cleaned up, we'll now force close
 		// the channel.