Your stomach will contain some nitrogen/oxygen swallowed while eating and some carbon dioxide from carbonated drinks (both of which is what will make you burp if the pressure gets too high). Same (to a lesser degree) for your intestines. However, the nitrogen/oxygen atmosphere will be gradually mixed with methane and traces of other stuff when traveling further down the digestive system, and loose oxygen. Some insect larvae are also capable of breathing from a bubble of air clinging to their behind if they are immersed in fluid. But I think the risk of any larvae surviving for long in your stomach will be low.
What you can do to make sure they die: Schnaps will cause more stomach acid to be released. And as always, chew properly.