x86/sev-es: Don't return NULL from sevesget_ghcb()
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 v5.12.7 by commit efbfc33b0b1c241311553d3c2229974ec1d684e2, it was introduced in version v5.10 by commit 0786138c78e79343c7b015d77507cbf9d5f15d00. For more details please see the references link.