encoding an out-of-range integer results in a corrupt message

Example:

```
var pb = require('protocol-buffers')
var m = pb("message Test1 { required int32 a = 1; }")
console.log(m.Test1.encode({a: 1e50}))
```

gives me: `<Buffer 08 80 80 80 80 80 80 80 80 80 80>` which looks like a corrupt message.

If you try to decode it, it hangs in an infinite loop.
