Merge pull request #556 from bjori/PHP-868-validate-mongodid

PHP-868: Method to check if string is a valid ObjectId

* bjori/PHP-868-validate-mongodid:
  PHP-868: Check string pointer and length in php_mongo_is_valid_id()
  PHP-868: Make php_mongo_is_valid_id() static and use const arg
  PHP-868: Method to check if string is a valid ObjectId
Don't pass null pointer to sasl_dispose() (Cristian H)