[mdns-avahi] use assert to make sure Service Resolver is valid (#1129)
[mdns-avahi] use assert to make sure Service Resolver is valid (#1129)

We used to use `assert` in `HandleResolveResult` to make sure the
Avahi resolver is a valid resolver, which make sense because only
valid resolvers can have callbacks. But there was a crash issue with
the original code so we changed to use `VerifyOrExit`. Turn out the
actual issue is that we should call `RemoveServiceResolver` as early
as possible so that the resolver won't be freed twice.
openthread/borderrouter,openthread/ot-br-posix,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/borderrouter,openthread/ot-br-posix