Fix wrong return value from LUKS2_generate_hdr on error.

On rare occasion (low memory) the function could return success
even though LUKS2 json metadata could be assembled properly.

lib/luks2/luks2_json_format.c

@@ -360,6 +360,7 @@ int LUKS2_generate_hdr(
 
 	if (json_object_object_add_by_uint(jobj_segments, 0, jobj_segment)) {
 		json_object_put(jobj_segment);
+		r = -ENOMEM;
 		goto err;
 	}