Bacported fix for CORE-3360: update ... returning ... raises -551 (no perm to update) for a column present only in the returning clause

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_1_Release@57765 65644016-39b1-43b1-bf79-96bc8fe82c15