[dns] fix potential discovery proxy crash (#867)
[dns] fix potential discovery proxy crash (#867)

This commit fixes potential Discovery Proxy crash when the mDNS
implementation replied with an empty host name.
openthread/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/ot-br-posix,openthread/ot-br-posix,openthread/borderrouter