fix panic error

[recall704]

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x7c0555]

goroutine 84 [running]:
github.com/ginuerzh/gost.(*sniHandler).Handle(0xc42028d520, 0xb05340, 0xc42000e468)
	/root/gopath/src/github.com/ginuerzh/gost/sni.go:65 +0x445
created by github.com/ginuerzh/gost.(*Server).Serve
	/root/gopath/src/github.com/ginuerzh/gost/server.go:62 +0x145
exit status 2

[f4nff]

@recall704 修正下#207

[f4nff]

@ginuerzh 修正下#207

[recall704]

@f4nff 自己学学自己修呗

[f4nff]

@recall704 臣妾做不到啊

[recall704]

@f4nff 我也做不到啊，边学边做呗。我们还要赚钱养家呢？哪有时间写(额外的)代码。