PDNS Provider Crashes

[ffledgling]

Reported by @fxpester on the kubernetes slack.

Verbatim:

---

tried pdns provider with new k8s 1.9.1
got such trace

panic: assignment to entry in nil map
 goroutine 1 [running]:
github.com/kubernetes-incubator/external-dns/provider.(*PDNSProvider).convertEndpointsToZones(0xc420199f90, 0xc420375fe0, 0x2, 0x2, 0x1735842, 0x7, 0x10, 0x156e4a0, 0x40eba2, 0xc420719f40, ...)
 /spare/local/ajaisingh/goworkspace/src/github.com/kubernetes-incubator/external-dns/provider/pdns.go:158 +0xa67
github.com/kubernetes-incubator/external-dns/provider.(*PDNSProvider).mutateRecords(0xc420199f90, 0xc420375fe0, 0x2, 0x2, 0x1735842, 0x7, 0x162ba20, 0xc420013630)
 /spare/local/ajaisingh/goworkspace/src/github.com/kubernetes-incubator/external-dns/provider/pdns.go:210 +0x84
github.com/kubernetes-incubator/external-dns/provider.(*PDNSProvider).ApplyChanges(0xc420199f90, 0xc42071a8a0, 0x1, 0xc420375f90)
 /spare/local/ajaisingh/goworkspace/src/github.com/kubernetes-incubator/external-dns/provider/pdns.go:270 +0x42f
github.com/kubernetes-incubator/external-dns/registry.(*NoopRegistry).ApplyChanges(0xc42026beb0, 0xc42071a8a0, 0x2, 0x2)
 /spare/local/ajaisingh/goworkspace/src/github.com/kubernetes-incubator/external-dns/registry/noop.go:44 +0x3d
github.com/kubernetes-incubator/external-dns/controller.(*Controller).RunOnce(0xc420118d40, 0xc420603bc8, 0x40eba2)
 /spare/local/ajaisingh/goworkspace/src/github.com/kubernetes-incubator/external-dns/controller/controller.go:64 +0x1c3
github.com/kubernetes-incubator/external-dns/controller.(*Controller).Run(0xc420118d40, 0xc42033a780)
 /spare/local/ajaisingh/goworkspace/src/github.com/kubernetes-incubator/external-dns/controller/controller.go:70 +0x40
main.main()
 /spare/local/ajaisingh/goworkspace/src/github.com/kubernetes-incubator/external-dns/main.go:165 +0xaba```

<hr />

This is likely due to a trailing dot handling bug in the `convertEndpointsToZones()` function