is giveaway.su safe ?

your acc wont get stolen, the service can't access your account credentials. Here what steam says when giveaway.su asks you to login with your account:
"By signing into authorization.su through Steam:
Your Steam login credentials will not be shared.
A unique numeric identifier will be shared with authorization.su. Through this, authorization.su will be able to identify your Steam community profile and access information about your Steam account according to your Profile Privacy Settings.
Any information on your Steam Profile page that is set to be publicly viewable may be accessed by authorization.su.
By clicking "Sign In" you agree to this data being shared."

Scam sites mimic this steamcommunity page and use similar urls like "stearncommunity", and thats how accounts get stolen, but in this case giveaway.su is legit, you really login in https://steamcommunity.com/

The login through Steam is safe (as long, as you are already logged in with Steam and don't enter your password for the 2nd time), but IIRC, it forces you to install an extension to your browser, and a browser extension can be a gateway for any kind of malware/keylogger.