Don't try to close the client config file if opening fails

$ q
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x9c124]

goroutine 1 [running]:
os.(*File).Close(0x0, 0xf800000010, 0x0, 0xf840054e40)
/usr/local/go/src/pkg/os/file_unix.go:99 +0x1c
github.com/miekg/dns.ClientConfigFromFile(0x1c3294, 0x6374652f00000010, 0x0, 0xf840054e40, 0xf840054de0, ...)
/Users/ask/go/src/github.com/miekg/dns/clientconfig.go:30 +0xcc
main.main()
/Users/ask/go/src/github.com/miekg/dns/ex/q/q.go:50 +0x45b

clientconfig.go

@@ -25,10 +25,10 @@ type ClientConfig struct {
 // a *ClientConfig.
 func ClientConfigFromFile(conf string) (*ClientConfig, error) {
 	file, err := os.Open(conf)
-	defer file.Close()
 	if err != nil {
 		return nil, err
 	}
+	defer file.Close()
 	c := new(ClientConfig)
 	b := bufio.NewReader(file)
 	c.Servers = make([]string, 0)