gsmi: fix null-deref in gsmi_get_variable

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.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/firmware/google/gsmi.c

@@ -361,9 +361,10 @@ static efi_status_t gsmi_get_variable(efi_char16_t *name,
 		memcpy(data, gsmi_dev.data_buf->start, *data_size);
 
 		/* All variables are have the following attributes */
-		*attr = EFI_VARIABLE_NON_VOLATILE |
-			EFI_VARIABLE_BOOTSERVICE_ACCESS |
-			EFI_VARIABLE_RUNTIME_ACCESS;
+		if (attr)
+			*attr = EFI_VARIABLE_NON_VOLATILE |
+				EFI_VARIABLE_BOOTSERVICE_ACCESS |
+				EFI_VARIABLE_RUNTIME_ACCESS;
 	}
 
 	spin_unlock_irqrestore(&gsmi_dev.lock, flags);