Fixed CORE-4488 - Wrong results of FOR SELECT <L> FROM <T> AS CURSOR <C> and table <T> is modified inside cursor's begin...end block.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@59898 65644016-39b1-43b1-bf79-96bc8fe82c15