First, Rice Krispies are fortified. The fortification process adds corn. ALWAYS. So if you see a product that is fortified with vitamins, just put it back on the shelf.
Fortification contains corn for two reasons.
1. The vitamins are sometimes sourced from corn as it the case with almost all Vitamin C products on the market. They are produced via fermentation of corn.
2. Corn and corn derivatives are used to stabilize the vitamins so that they are viable when added to the product, and so they are shelf-stable.. so that when you eat the product the vitamins are still viable.
Also in the original Rice Krispies there is malt flavor. There are two things this could be and both are so often from corn that I'd be surprised if it wasn't.
1. Malt Flavor can be from actual malted products, which is usually a mix of corn and barley. That it's not included in the gluten-free one, means there's likely some barley in it.
2. "Flavor" in most products - even natural flavor - is made in a lab somewhere from chemicals, which are usually corny.
All this to say, yes... Rice Krispies are not safe for corn allergies.