net: dns: mdns_responder: Interface name might miss terminating null

The network interface name that is copied to if_req struct might
be missing terminating null for IPv4.

This is fixing the IPv4 issue which was missed in previous fix attempt.

Fixes #74795
Coverity-CID: 368797

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>