panic when num conns == 0

```
panic: invalid argument to Intn

goroutine 62 [running]:
runtime.panic(0x846860, 0xc2108b9fc0)
        /usr/local/go/src/pkg/runtime/panic.c:266 +0xb6
math/rand.(*Rand).Intn(0xc21000a160, 0x0, 0x1)
        /usr/local/go/src/pkg/math/rand/rand.go:89 +0x65
math/rand.Intn(0x0, 0xc21012f120)
        /usr/local/go/src/pkg/math/rand/rand.go:155 +0x34
github.com/hailocab/go-nsq.(*Reader).rdyLoop(0xc2100d3200)
        /mnt/tmp/EO6AEmKY_hailocab-go-login-service-ac74f36abe66/src/github.com/hailocab/go-nsq/reader.go:660 +0xf9
created by github.com/hailocab/go-nsq.NewReader
        /mnt/tmp/EO6AEmKY_hailocab-go-login-service-ac74f36abe66/src/github.com/hailocab/go-nsq/reader.go:193 +0x3ec
```