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.