I went there on the same version of Opera but I didn't get that alert. The reason though is because Ghostery blocks that tracker. So, I unblocked it temporarily and refreshed the page and got the same popup.
I wouldn't worry about it. Opera 12x (and 11x) like to be very picky about security certs. Opera is correct that the server's cert doesn't match its host name. But since that is just a tracking site that you are letting track you has this problem doesn't really matter because you are not going to linkmaker.itunes.apple.com but just passing by on your way to The Sydney Morning Herald.
I tried Pale Moon there just now and Ghostery blocked 11 trackers including the itunes one. So, I told Ghostery to allow it. I refreshed the page and no cert popup. But I didn't expect Fx or Pale Moon to alert as only Opera before version 15 was so minuté about certs. Once I stopped using Opera as my default browser, I still check anything odd about a cert using Opera because it will tell me things that other browsers don't. Sometimes I want to know those things but other times its more of "nuisance" like in this case