efi_loader: fix an IS_ERR() vs NULL check

The efi_parse_pkcs7_header() function returns NULL on error so the check
for IS_ERR() should be changed to a NULL check.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/lib/efi_loader/efi_capsule.c b/lib/efi_loader/efi_capsule.c
index ddf8153..af8a2ee 100644
--- a/lib/efi_loader/efi_capsule.c
+++ b/lib/efi_loader/efi_capsule.c
@@ -368,9 +368,8 @@
 					     auth_hdr->auth_info.hdr.dwLength
 					     - sizeof(auth_hdr->auth_info),
 					     &buf);
-	if (IS_ERR(capsule_sig)) {
+	if (!capsule_sig) {
 		debug("Parsing variable's pkcs7 header failed\n");
-		capsule_sig = NULL;
 		goto out;
 	}