efi_loader: efi_variable_parse_signature() returns NULL on error

efi_variable_parse_signature() returns NULL on error, so IS_ERR()
is an incorrect check.  The goto err leads to pkcs7_free_message(),
which works fine on a NULL ptr.

Signed-off-by: Patrick Wildt <patrick@blueri.se>
Reviewed-by: AKASHI Takahiro <takahiro.akashi@linaro.org>