db_mongodb: fix warning for bool comparison

- reported by Olle E. Johansson, GH #696

modules/db_mongodb/mongodb_dbase.c

@@ -1204,7 +1204,7 @@ int db_mongodb_update(const db1_con_t* _h, const db_key_t* _k,
 		if(db_mongodb_bson_add(sdoc, _uk[i], _uv+i, i)<0)
 			goto error;
 	}
-	if(bson_append_document(udoc, "$set", 4, sdoc)<0) {
+	if(!bson_append_document(udoc, "$set", 4, sdoc)) {
                 LM_ERR("failed to append document to bson document\n");
                 goto error;
         }