BUG/MINOR: resolvers: accept fields at the response boundary

BUG/MINOR: resolvers: accept fields at the response boundary

resolv_validate_dns_response() treats <bufend> as an exclusive end
pointer. Its fixed-width field checks nevertheless reject
"reader + 2 == bufend", although reader[0] and reader[1] are both
within the response.

Use a strict greater-than comparison for every two-byte DNS header and
question field. No complete successful answer is affected, but this
corrects the parser contract and lets later checks classify minimal
responses instead of rejecting them early as invalid.

This may be backported as far as 2.4.