fix panic when interface up but hasn't got dhcp address yet

[robphoenix]

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x402fd7]

goroutine 1 [running]:
panic(0x624da0, 0xc42000c130)
	/usr/local/go/src/runtime/panic.go:500 +0x1a1
main.New(0x7, 0x5dc, 0xc42007cfe0, 0xf, 0xc4200f3014, 0x6, 0x8ec, 0x13, 0xc4200742a0, 0x0, ...)
	/home/rob/code/go/src/github.com/robphoenix/trawl/trawl.go:45 +0x1d7
main.main()
	/home/rob/code/go/src/github.com/robphoenix/trawl/main.go:141 +0x76f