net: mdns + dns_sd: fix regression that breaks ptr queries

While adding support for service type enumeration, a regression was
introduced which prevented mDNS ptr query responses.

1. There was an off-by-one error with label size checking
2. Valid queries were failing to match in `dns_rec_match()` due to
   not checking for either NULL or 0 "wildcard" port

Fixes #39284

Signed-off-by: Christopher Friedt <chrisfriedt@gmail.com>