[PHP] Replace default $flags variable with 0

[iricketson]

The sfValidatorIp class sets the $flags variable to null by default, which is passed into the $options parameter of the filter_var function and triggers a PHP deprecation error:

Deprecated: filter_var(): Passing null to parameter #3 ($options) of type array|int is deprecated in /path/to/friendsofsymfony1/symfony1/lib/validator/sfValidatorIp.class.php on line 104

This PR simply sets the default value of $flags to 0 (zero), which is the default filter_var $options value when not passed.