Backported fix for CORE-4566: Incorrect size of the output parameter/argument when execute block, procedure or function use system field in metadata charset

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_5_Release@60084 65644016-39b1-43b1-bf79-96bc8fe82c15