Anal nitrite appears to be a classical malapropism rather than an eggcorn. Eggcorns make plausible sense in the same context as the original phrase - they are usually more sensible than the original, in the mind of the eggcorn-user. For anal nitrite to be an eggcorn, users would have to believe that it has something to do with anus. For amyl nitrate to be an eggcorn, users would have to believe that the compound contains a nitrate ion, and have some idea of what that means.