mdns: fix possible crash when probing on particular interface with duplicated service instances due to naming conflicts on network
Issue: MDNS server initially sends probing packets to resolve naming confilicts with already registered service instances. In case of a conflict, instance name is altered and probing restarts. Original instance however wasnnot removed from the structure and upon service removal only one entry was removed and a dangling service might have been kept in the structure to bring about a crash.
Resolution: Keep only one instance of a service in the probing structure.