what if I clear CAT as well as interview, and get selected, but my last semester of graduation results are not out?

Shubhi, Final year Graduates are given admission provisionally. Which means, by the time of admission, if the final year results have not been declared yet, you will have to get a letter from the Head of your Institution that you are a student at this Institution and as on the date of the letter issued, you fulfil the eligibility criteria for admission on the basis of CAT. Once your results have been declared and you get your documents, you will submit them in the MBA College and your admission will be finalised. Until then your admission will be provisional subjective to your final year results.

Hello student cat selection process begins in the month of May June by that time all final semester results are out in every college that's why cat eligibility is a final year graduate student are eligible.
Nothing to worry just do you best for cat exam and start preparing for personal interview after that.
You can also use free mock test for cat by careers360 which is very helpful to revise your concept and grasp the things.
https://learn.careers360.com/test-series-cl-cat-2019/?icn=CAT_test_series&ici=CAT_article_contmap

Feel free to comment if you have any doubt
Good luck