BUG/MINOR: resolvers: fix risk of appending garbage past the domain name

BUG/MINOR: resolvers: fix risk of appending garbage past the domain name

The previous fix 75f72c2eb ("BUG/MEDIUM: resolvers: Fix test on dn label
size in resolv_dn_label_to_str()") may still leave garbage from the input
buffer into the response: if a component length is passed as zero, it
should mark the end, but instead a dot will be emitted, and whatever
follows it in the input buffer would continue to be appended as extra
components. While having no direct consequences beyond the domain not
being properly decoded, it could at least complicate troubleshooting.

This should be backported where the fix above is backported.