[mdns-avahi] fix potential crash on browsing (#1121)
[mdns-avahi] fix potential crash on browsing (#1121)

This commit fixes a bug that `aName` is `nullptr` when `aEvent` is not
`AVAHI_BROWSER_NEW`. So, we should only use `aName` on
`AVAHI_BROWSER_NEW`.
openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/ot-br-posix,openthread/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/ot-br-posix,openthread/borderrouter,openthread/borderrouter,openthread/borderrouter