can: ctucanfd: handle skb allocation failure

In the Linux kernel, the following vulnerability has been resolved: can: ctucanfd: handle skb allocation failure If skb allocation fails, the pointer to struct can_frame is NULL. This is actually handled everywhere inside ctucan_err_interrupt() except for the only place. Add the missed NULL check. Found by Linux Verification Center (linuxtesting.org) with SVACE static analysis tool.