gsmi: fix null-deref in gsmi_get_variable

commit a769b05e upstream.

We can get EFI variables without fetching the attribute, so we must
allow for that in gsmi.

commit 85974825 ("efi: pstore: Omit efivars caching EFI varstore
access layer") added a new get_variable call with attr=NULL, which
triggers panic in gsmi.

Fixes: 74c5b31c

 ("driver: Google EFI SMI")
Cc: stable <stable@kernel.org>
Signed-off-by: Khazhismel Kumykov <khazhy@google.com>
Link: https://lore.kernel.org/r/20230118010212.1268474-1-khazhy@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>