Fix flaky swarm/network/stream test

[nonsense]

ok  	github.com/ethereum/go-ethereum/swarm/network/simulations/discovery	39.219s	coverage: 0.0% of statements
ERROR[01-24|13:12:09.898|swarm/network/stream/delivery.go:266] Delivery.RequestFromPeers: peer doesn't have stream cap. we should have returned at sp == nil 
ERROR[01-24|13:12:18.900|swarm/network/stream/peer.go:91]      Message send error, dropping peer        peer=8c4b7e792a398252 err="shutting down"
ERROR[01-24|13:12:18.900|swarm/network/stream/peer.go:91]      Message send error, dropping peer        peer=8c4b7e792a398252 err="shutting down"
ERROR[01-24|13:12:18.900|swarm/network/stream/peer.go:91]      Message send error, dropping peer        peer=8c4b7e792a398252 err="shutting down"
ERROR[01-24|13:12:18.900|swarm/network/stream/peer.go:91]      Message send error, dropping peer        peer=8c4b7e792a398252 err="shutting down"
ERROR[01-24|13:12:18.901|swarm/network/stream/peer.go:91]      Message send error, dropping peer        peer=8c4b7e792a398252 err="write tcp 127.0.0.1:55236->127.0.0.1:39817: use of closed network connection"
ERROR[01-24|13:12:18.901|swarm/network/stream/peer.go:91]      Message send error, dropping peer        peer=8c4b7e792a398252 err="shutting down"
ERROR[01-24|13:12:18.901|swarm/network/stream/peer.go:91]      Message send error, dropping peer        peer=8c4b7e792a398252 err="shutting down"
ERROR[01-24|13:12:19.164|swarm/network/stream/peer.go:91]      Message send error, dropping peer        peer=87cb0d94debff391 err="shutting down"
ERROR[01-24|13:12:19.164|swarm/network/stream/peer.go:91]      Message send error, dropping peer        peer=87cb0d94debff391 err="shutting down"
ERROR[01-24|13:12:19.164|swarm/network/stream/peer.go:91]      Message send error, dropping peer        peer=87cb0d94debff391 err="shutting down"
ERROR[01-24|13:12:19.164|swarm/network/stream/peer.go:91]      Message send error, dropping peer        peer=87cb0d94debff391 err="shutting down"
ERROR[01-24|13:12:19.164|swarm/network/stream/peer.go:91]      Message send error, dropping peer        peer=87cb0d94debff391 err="write tcp 127.0.0.1:59640->127.0.0.1:35520: use of closed network connection"
ERROR[01-24|13:12:19.164|swarm/network/stream/peer.go:91]      Message send error, dropping peer        peer=87cb0d94debff391 err="shutting down"
ERROR[01-24|13:12:19.164|swarm/network/stream/peer.go:91]      Message send error, dropping peer        peer=87cb0d94debff391 err="shutting down"
ERROR[01-24|13:12:19.951|swarm/network/stream/delivery.go:266] Delivery.RequestFromPeers: peer doesn't have stream cap. we should have returned at sp == nil 
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x461264]
goroutine 159 [running]:
github.com/ethereum/go-ethereum/swarm/network.(*Fetcher).doRequest(0xc422065f80, 0xc6ff60, 0xc421c155c0, 0xc4213d4de0, 0xc42025e8a0, 0x0, 0x0, 0x0, 0xc6ff01, 0x0, ...)
	/home/travis/gopath/src/github.com/ethereum/go-ethereum/swarm/network/fetcher.go:307 +0x27a
github.com/ethereum/go-ethereum/swarm/network.(*Fetcher).run(0xc422065f80, 0xc6ff60, 0xc421c155c0, 0xc42025e8a0)
	/home/travis/gopath/src/github.com/ethereum/go-ethereum/swarm/network/fetcher.go:231 +0x4ee
created by github.com/ethereum/go-ethereum/swarm/network.(*FetcherFactory).New
	/home/travis/gopath/src/github.com/ethereum/go-ethereum/swarm/network/fetcher.go:114 +0x13c
FAIL	github.com/ethereum/go-ethereum/swarm/network/stream	10.130s

[janos]

This issue should be fixed with ethereum/go-ethereum#18971.