json.c: improve ast_json_to_ast_variables performance

When converting from a json object to an ast variables list the conversion
algorithm was doing a complete traversal of the entire variables list for
every item appended from the json structure.

This patch makes it so the list is no longer traversed for each new ast
variable being appended.

Change-Id: I8bf496a1fc449485150d6db36bfc0354934a3977

main/json.c

@@ -1046,6 +1046,7 @@ struct ast_json *ast_json_party_id(struct ast_party_id *party)
 enum ast_json_to_ast_vars_code ast_json_to_ast_variables(struct ast_json *json_variables, struct ast_variable **variables)
 {
 	struct ast_json_iter *it_json_var;
+	struct ast_variable *tail = NULL;
 
 	*variables = NULL;
 
@@ -1082,7 +1083,7 @@ enum ast_json_to_ast_vars_code ast_json_to_ast_variables(struct ast_json *json_v
 			return AST_JSON_TO_AST_VARS_CODE_OOM;
 		}
 
-		ast_variable_list_append(variables, new_var);
+		tail = ast_variable_list_append_hint(variables, tail, new_var);
 	}
 
 	return AST_JSON_TO_AST_VARS_CODE_SUCCESS;