BUG/MEDIUM: resolvers: reject too long labels when encoding a name

BUG/MEDIUM: resolvers: reject too long labels when encoding a name

A 256-byte label passed through do-resolve (e.g. from hdr(host)) wraps
its encoded length to zero. Callers using strdup() then truncate the
name but retain its full length, causing out-of-bounds reads and heap
data disclosure in DNS queries. Embedded NULs cause the same issue.

Reject labels longer than DNS_MAX_LABEL_SIZE and embedded NULs in
resolv_str_to_dn_label(), before callers duplicate the encoded name.

This affects do-resolve since its introduction in 2.0 by commit
333939c2e ("MINOR: action: new '(http-request|tcp-request content)
do-resolve' action"). Must be backported to all stable branches.