Fix a bug that deleted too many hosts

When running a command that takes host ranges as arguments (e.g.,
`hosts`, `services`), the arguments get parsed by
Rex::Socket::RangeWalker. If RangeWalker was unable to parse, it would
return nil, which in this context means "all hosts." If the user is
searching, they get all hosts instead of the ones they were interested
in -- this is annoying, but not too big a deal. Unfortunately, the same
logic applied when *deleting* hosts, with `hosts -d ...`, causing all
hosts to be deleted when giving it an invalid range.