Simplify condition for running stripslashes

- get_magic_quotes_gpc is always present in supported PHP versions
- get_magic_quotes_gpc returns false since PHP 5.4.0 where related
  setting has been removed

Signed-off-by: Michal Čihař <michal@cihar.com>
Add test for double call of PMA_arrayWalkRecursive

Signed-off-by: Michal Čihař <michal@cihar.com>