panic: runtime error: invalid memory address or nil pointer dereference

For

```
deploy:
image: appleboy/drone-ssh
host: example.dev
user: nemonik
port: 22
key: ${SSH_KEY}
script:
- echo "Hey"
- ls
when:
branch:
include: [ master, release/* ]
```

After registering my SSH_KEY as a secret via drone cli

```
drone secret add --image=appleboy/drone-ssh Ephemeral/nodejs-helloworld SSH_KEY "$(cat ~/.ssh/privatekey)"
```

I get out of drone:

```
2017/02/09 05:15:32 example.dev commands: echo "Hey"
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x54e043]
goroutine 18 [running]:
panic(0x62b220, 0xc42000a060)
/usr/local/go/src/runtime/panic.go:500 +0x1a1
github.com/appleboy/drone-ssh/vendor/golang.org/x/crypto/ssh.publicKeyCallback.auth(0xc42007f560, 0xc420010d40, 0x20, 0x20, 0xc42006cb10, 0x7, 0x772a00, 0xc4200e8000, 0x770600, 0xc4200705d0, ...)
/srv/app/src/github.com/appleboy/drone-ssh/vendor/golang.org/x/crypto/ssh/client_auth.go:191 +0xb3
github.com/appleboy/drone-ssh/vendor/golang.org/x/crypto/ssh.(*connection).clientAuthenticate(0xc4200dc100, 0xc4200e0000, 0x0, 0xa)
/srv/app/src/github.com/appleboy/drone-ssh/vendor/golang.org/x/crypto/ssh/client_auth.go:34
github.com/appleboy/drone-ssh/vendor/golang.org/x/crypto/ssh.(*connection).clientHandshake(0xc4200dc100, 0xc420070db0, 0x22, 0xc4200e0000, 0x0, 0x0)
/srv/app/src/github.com/appleboy/drone-ssh/vendor/golang.org/x/crypto/ssh/client.go:107 +0x2ed
github.com/appleboy/drone-ssh/vendor/golang.org/x/crypto/ssh.NewClientConn(0x774b20, 0xc42002a008, 0xc420070db0, 0x22, 0xc420069a90, 0x774b20, 0xc42002a008, 0x0, 0x0, 0xc42007f560, ...)
/srv/app/src/github.com/appleboy/drone-ssh/vendor/golang.org/x/crypto/ssh/client.go:75 +0x105
github.com/appleboy/drone-ssh/vendor/golang.org/x/crypto/ssh.Dial(0x666e8c, 0x3, 0xc420070db0, 0x22, 0xc420069a90, 0x22, 0x0, 0x1)
/srv/app/src/github.com/appleboy/drone-ssh/vendor/golang.org/x/crypto/ssh/client.go:171 +0xb3
github.com/appleboy/drone-ssh/easyssh.(*MakeConfig).connect(0xc420069e90, 0x45c000, 0x10000c42007c010, 0x0)
/srv/app/src/github.com/appleboy/drone-ssh/easyssh/easyssh.go:77 +0x16b
github.com/appleboy/drone-ssh/easyssh.(*MakeConfig).Stream(0xc420069e90, 0xc42006cbf0, 0xd, 0xd, 0xd, 0xc420069ca8, 0x4431ce)
/srv/app/src/github.com/appleboy/drone-ssh/easyssh/easyssh.go:95 +0x40
github.com/appleboy/drone-ssh/easyssh.(*MakeConfig).Run(0xc420069e90, 0xc42006cbf0, 0xd, 0x666c32, 0x1, 0xc42006cbf0, 0xd)
/srv/app/src/github.com/appleboy/drone-ssh/easyssh/easyssh.go:130 +0x5d
main.Plugin.Exec.func1(0xc42006cac6, 0xa, 0x0, 0x0, 0xc42006cb10, 0x7, 0x0, 0x0, 0xc42006c940, 0x1, ...)
/srv/app/src/github.com/appleboy/drone-ssh/plugin.go:71 +0x330
created by main.Plugin.Exec
/srv/app/src/github.com/appleboy/drone-ssh/plugin.go:79 +0x16e
```