Somewhat hackery fix for CORE-4201: Computed field returns null value inside BI trigger.
A better solution requires major changes and will be investigated in trunk later.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_5_Release@58646 65644016-39b1-43b1-bf79-96bc8fe82c15