Freezer bags are made of polyethylene, a food grade plastic designed to take heat.
One shouldn't be burning any garbage really....when in the backcountry. Most commercial meal bags are lined btw (be it MH or other brands), the outer shiny mylar like packaging acts as a cozy, the inner lining ensures you don't get taste crossover from the packaging. Old school MH packages used to have the food in a plastic sack, that you slipped a cardboard collar over to seal, then tucked into the metallic outer bag.
Anyhow, point is...when it comes to bags, roll them up tightly (like a cigarette), seal and toss in a garbage sack for the trip.