xen/acpi: Fix potential memory leak.

Coverity points out that we do not free in one case the
pr_backup - and sure enough we forgot.

Found by Coverity (CID 401970)
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xen-acpi-processor.c

@@ -520,15 +520,18 @@ static int __init xen_acpi_processor_init(void)
 
 		if (!pr_backup) {
 			pr_backup = kzalloc(sizeof(struct acpi_processor), GFP_KERNEL);
+			if (pr_backup)
 				memcpy(pr_backup, _pr, sizeof(struct acpi_processor));
 		}
 		(void)upload_pm_data(_pr);
 	}
 	rc = check_acpi_ids(pr_backup);
-	if (rc)
-		goto err_unregister;
 
 	kfree(pr_backup);
+	pr_backup = NULL;
+
+	if (rc)
+		goto err_unregister;
 
 	return 0;
 err_unregister: