Do not allow undefined peerInfo

# What

The code assumes that `peerInfo` exists, the API doesn't.

# How to test

```js
swarm = new Swarm()
swarm.addTransport('tcp', tcp, { multiaddr: mh }, {}, {port: 8095})
```

This shouldn't explode because of `self.peerInfo.multiaddrs`.

src/swarm.js

@@ -11,6 +11,10 @@ function Swarm (peerInfo) {
     throw new Error('Swarm must be called with new')
   }
 
+  if (!peerInfo) {
+    throw new Error('You must provide a value for `peerInfo`')
+  }
+
   self.peerInfo = peerInfo
 
   // peerIdB58: { conn: <conn> }