Fix nil map access of endpoint labels

[shashidharatd]

We are seeing crash in latest code as below

panic: assignment to entry in nil map

goroutine 1 [running]:
github.com/kubernetes-incubator/external-dns/registry.(*TXTRegistry).Records(0xc0001025b0, 0xc000000300, 0xc0006ed828, 0xc0004ca8a0, 0xc0004ca8a0, 0xc0006ed828)
	/go/src/github.com/kubernetes-incubator/external-dns/registry/txt.go:103 +0x49a
github.com/kubernetes-incubator/external-dns/controller.(*Controller).RunOnce(0xc0004c0b80, 0xc0006ed7f8, 0x2)
	/go/src/github.com/kubernetes-incubator/external-dns/controller/controller.go:67 +0x49
github.com/kubernetes-incubator/external-dns/controller.(*Controller).Run(0xc0004c0b80, 0xc000086960)
	/go/src/github.com/kubernetes-incubator/external-dns/controller/controller.go:100 +0x84
main.main()
	/go/src/github.com/kubernetes-incubator/external-dns/main.go:240 +0xb0e

probably this was introduced by #713

/cc @njuettner @linki

[njuettner]

Thanks for your PR @shashidharatd 👍
We probably oversaw this case, nice catch!

[njuettner]

/lgtm

[njuettner]

/approve

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