isdn: cpai: check ctr->cnr to avoid array index out of bound
This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven. This ID is fixed in Linux Kernel version v4.19.214 by commit 7d91adc0ccb060ce564103315189466eb822cc6a. For more details please see the references link.