fix error when cert or key is nil

[auhlig]

If the key ("tls.crt"|"tls.key") in secret.Data map exists, but the value is nil the ingress controller crashes. Check this and return an error if this is the case to prevent:

...
backend_ssl.go:64] adding secret <ns>/<sn> to the local store
runtime.go:66] Observed a panic: "invalid memory address or nil pointer dereference" (runtime error: invalid memory address or nil pointer dereference)

[k8s-reviewable]

This change is 

[coveralls]

Coverage decreased (-0.03%) to 43.686% when pulling 003667f on auhlig:nilpointerfix into 37bd14d on kubernetes:master.

[aledbf]

/lgtm

[aledbf]

@auhlig thanks!