Like many of the 'do not feed' things the level of toxicity of salt is not as much of a concern as some pretend it to be moderation is key...
You would have to feed the chickens high levels of salt to cause any real harm, an occasional and moderate dosage of salt isn't any more harmful then a chicken that free ranges in a coastal area where salt is all over everything... It's all about moderation, a little extra salt here and there isn't going to cause any harm, especially if they have plenty of fresh water to wash it out of their systems...
It's when you have high levels of sale and/or limited fresh water or a diet constantly high in salt that you see problems...
This is a horrible scan but it still is readable...
https://archive.org/stream/toxicityofsaltfo00mitc/toxicityofsaltfo00mitc_djvu.txt
If you gloss over it you will see that chickens given 4 grams of salt twice daily appeared to do fine, that is a lot of salt, this is about about 1.5 teaspoons of pure salt a day!
With that said and the average adult chicken being about 4.5lbs or 2kg it would take about a tablespoon (8-10 grams) sized does of pure salt to be lethal...
I highly doubt the chickens could consume enough rice crackers to get a lethal dose of salt...