If you must get a degree (I highly recommend skipping it and going straight into the work place) go for something like Economics and/or Business.
Understanding how businesses work and understanding economies is very important to be successful as an IMer. If you don't naturally know this, then learn it at Uni.
I understand that you want a backup. But trust me, unless you're studying towards a technical degree like Engineering (which will in no way help you with IM) then you are just whittling money away worthlessly. Almost all other degrees don't get you jobs. You think you'll get a job, then you come out of uni and realize that, of shit, 1000 other people have my degree. Why would a company hire me, a business graduate, when they can hire this other person who has had experience running businesses?
Thats the thing. Outside of technical degrees, you might as well go out and get solid work experience instead. When you write a CV, do you know what you put first and foremost? Work experience. Do you know what gets delegated to the bottom in a corner hidden away? "Qualifications". No one cares. They want proof that you can do their job. And that comes from work experience.
So if you want a backup, either go for a completely non-IM field, or else split your time between IM and pursueing a RL job. Use your time you'd have spent at uni getting work experience instead.