I don't think Bilbo's last "riddle", which was a question, and not a riddle, was fair if we're talking just about the game they're playing. However, Bilbo was fighting for his life in this game and I think we would all have done the same to escape Gollum -Smeagol. We would have done the same to survive.
Bilbo also starts becoming aware around this time that the ring belonged to Gollum and that it's not just a nice piece of jewelry.
In fairness I have to point out that Gollum gives Bilbo an opening to ask this last "riddle" because he does say something like "ask me a question!!" when Bilbo is taking too long to come up with a new riddle. So, on Bilbo's side, he's really responding in the right way to what Gollum just asked him to do.
Bilbo would have made a great lawyer.
Gollum and Bilbo screencap from
http://penningtonplanetarium.files.wordpress.com/2013/09/bilbosmeagol.jpg
