efi: Only register shim_lock verifier if shim_lock protocol is found and SB enabled

efi: Only register shim_lock verifier if shim_lock protocol is found and SB enabled

The shim_lock module registers a verifier to call shim's verify, but the
handler is registered even when the shim_lock protocol was not installed.

This doesn't cause a NULL pointer dereference in shim_lock_write() because
the shim_lock_init() function just returns GRUB_ERR_NONE if sl isn't set.

But in that case there's no point to even register the shim_lock verifier
since won't do anything. Additionally, it is only useful when Secure Boot
is enabled.

Finally, don't assume that the shim_lock protocol will always be present
when the shim_lock_write() function is called, and check for it on every
call to this function.

Reported-by: Michael Chang <mchang@suse.com>
Reported-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>