Missing notification of gc thread.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@59958 65644016-39b1-43b1-bf79-96bc8fe82c15
Restored a check dropped accidentially by one of my recent commits

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@59959 65644016-39b1-43b1-bf79-96bc8fe82c15