"Super" might be short for "superintendent". I haven't looked at the film. Remember your modals of deduction? :) "You must have saved it..." means "I'm sure that you saved it..."

In the United States... at least in cities in the Northeast (such things are regional!) "the super" is the usual short name for "the building superintendent." The superintendent usually lives in the building, works for the landlord who owns the building, is available most of the time on call, and takes care of day-to-day things. The super has the master key and if the police needed to get into an apartment the first they'd do is "see the super." The remark about "saved it for a good reason" is complicated and part joking. Without context I'm not sure I understand it. In the United States we believe there is a Chinese tradition that if you save somebody's life, you are then responsible for taking care of them afterward. (I don't know if this is really true!) The second person might have been saying "if you didn't want to take care of me, you shouldn't have saved my life." Taking that a step farther, "Unless you had some reason why you wanted to take care of me, you shouldn't have saved my life." It's definitely a joke because of course you would be grateful to anybody who saved your life. It sounds like a way of saying "we're buddies now," we're in this together.