This case wasn't properly handled. Simply return nil.
It could also return 127.0.0.1 or 127.0.0.0/8 instead
of early exit though I doubt it makes that much sense
for user.
Scripts no longer need to implement caching of datafiles tables in the
registry, since the datafiles.lua library keeps its own cache in the
registry. A side-effect is that scripts should not change the tables
returned by datafiles.parse_{protocols,rpc,services,mac_prefixes}(), as
doing so will affect all other scripts that use those functions.
