fix compat with PHP 5.3

When running test suite against PHP 5.3.3 (RHEL-6)

      PHP Fatal error:  Using $this when not in object context in /builddir/build/BUILD/php-horde-Horde-Db-2.2.0/Horde_Db-2.2.0/lib/Horde/Db/Adapter/Base.php on line 632

I think older PHP doesn't allow to use this inside an anonymous function.

This simple fix restore PHP 5.3 compatibility (supported, according to package.xml min version)