fix connErr panic while NewClient fail

I use the driver and write a simple example like this:

```
package main

import (
	"fmt"
	kafka "github.com/yqylovy/sarama"
)

func main() {
	// kafka.NewClient("my_client1", []string{"192.168.10.28:9092"}, nil)
	_, err := kafka.NewClient("my_client1", []string{"192.168.10.28:9092"}, nil)
	if err != nil {
		fmt.Println("A ERROR:" + err.Error())
		return
	} else {
		fmt.Println("> connected")
		// defer client.Close()
	}
}
```

however,it panic while I run it 

```
panic: runtime error: invalid memory address or nil pointer dereference
	[signal 0xb code=0x1 addr=0x18 pc=0x439e53]
	
	goroutine 1 [running]:
	runtime.panic(0x535640, 0x6e0b68)
		/usr/local/go/src/pkg/runtime/panic.c:266 +0xb6
	github.com/Shopify/sarama.(*Broker).send(0xc210059120, 0x56d5b0, 0xa, 0x7f6b1bdf02b8, 0xc21004d180, ...)
		/usr/share/go/src/github.com/Shopify/sarama/broker.go:275 +0x243
	github.com/Shopify/sarama.(*Broker).sendAndReceive(0xc210059120, 0x56d5b0, 0xa, 0x7f6b1bdf02b8, 0xc21004d180, ...)
		/usr/share/go/src/github.com/Shopify/sarama/broker.go:297 +0x7a
	github.com/Shopify/sarama.(*Broker).GetMetadata(0xc210059120, 0x56d5b0, 0xa, 0xc21004d180, 0x1, ...)
		/usr/share/go/src/github.com/Shopify/sarama/broker.go:171 +0xa2
	github.com/Shopify/sarama.(*Client).refreshMetadata(0xc21005c000, 0x6ed000, 0x0, 0x0, 0x3, ...)
		/usr/share/go/src/github.com/Shopify/sarama/client.go:279 +0x21b
	github.com/Shopify/sarama.(*Client).RefreshAllMetadata(0xc21005c000, 0x0, 0x0)
		/usr/share/go/src/github.com/Shopify/sarama/client.go:192 +0x65
	github.com/Shopify/sarama.NewClient(0x56d5b0, 0xa, 0xc21000a3f0, 0x1, 0x1, ...)
		/usr/share/go/src/github.com/Shopify/sarama/client.go:67 +0x39d
	main.main()
		/root/workspace/src/test/test_kafka2.go:11 +0xaf
```

So I modify the connErr judgement to make it avoid panic and output  `kafka: Client has run out of available brokers to talk to. Is your cluster reachable?`