network: avoid looping forever

blockFetcherFin is closed by the block fetcher, so it will forever return
something in this loop making it an infinite loop, creating useless load
and affecting normal node operation.

Signed-off-by: Roman Khimov <roman@nspcc.ru>

pkg/network/server.go

@@ -575,6 +575,7 @@ func (s *Server) run() {
 				s.tryInitStateSync()
 				s.tryStartServices()
 			}
+			s.blockFetcherFin = nil
 		}
 	}
 }