Fix crash when reading dns:// without a hostname

Previously, `read dns://` crashed the interpreter. With this fix, a
dns:// spec without a hostname (or IPv4 address) is considered invalid
for READ access:

    >> read dns://
    ** Access error: invalid spec or options: dns://

Fixes the crash reported in CureCode issue #1935.
Merge pull request #66 from earl/fix-empty-dns

Fix crash when reading dns:// without a hostname