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>
diff --git a/lib/efi_loader/efi_variable.c b/lib/efi_loader/efi_variable.c
index 568bd3c..60c1201 100644
--- a/lib/efi_loader/efi_variable.c
+++ b/lib/efi_loader/efi_variable.c
@@ -524,9 +524,8 @@
 	var_sig = efi_variable_parse_signature(auth->auth_info.cert_data,
 					       auth->auth_info.hdr.dwLength
 						   - sizeof(auth->auth_info));
-	if (IS_ERR(var_sig)) {
+	if (!var_sig) {
 		debug("Parsing variable's signature failed\n");
-		var_sig = NULL;
 		goto err;
 	}