I agree with Hilary - discard it.

Should you find yourself in a meat predicament again, askkaren.gov is another good resource. Useful for difficult food safety questions.

Here's what the FDA says:
"If the packaging material remains unaltered, that is unmelted and didn't come apart the cooked meat will not pose imminent health hazard. If the package has changed shape, altered or melted in someway do not use the product."
Since this is stock, it is my opinion that you've given the material time to really stew and get in your stock. I would discard it. Here is the link from the FDA, should you be interested.
http://www.fsis.usda.gov/wps/wcm/connect/21478ad0-4e14-4dfd-b822-952d08997a9b/Hock_Locks_and_Other_Accoutrements.pdf?MOD=AJPERES

I meant - no big thing to throw it out. It's only stock, not hard to replace.

No, not OK. You boiled plastic and chemicals with your chicken. No big thing - it's only stock. You can make more another time.