KEYS: prevent NULL pointer dereference in find_asymmetric_key()

In the Linux kernel, the following vulnerability has been resolved: KEYS: prevent NULL pointer dereference in find_asymmetric_key() In find_asymmetric_key(), if all NULLs are passed in the id_{0,1,2} arguments, the kernel will first emit WARN but then have an oops because id_2 gets dereferenced anyway. Add the missing id_2 check and move WARN_ON() to the final else branch to avoid duplicate NULL checks. Found by Linux Verification Center (linuxtesting.org) with Svace static analysis tool.