Exception: NoMethodError: undefined method `name' for nil:NilClass - ensure status of image details is OK before outputting image name

This patch fixes a problem when issuing  "knife digital_ocean droplet list"

I believe the reason for this error is that old images no longer available will return a status ERROR when queried and a nil is dropped.

See also: https://github.com/rmoriz/knife-digital_ocean/issues/18 and https://github.com/rmoriz/knife-digital_ocean/issues/15 

Thanks for the useful tool!