Fixed memory leak followed by AV (introduced at fix for CORE-3989).
Dmitry, please review.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_5_Release@58188 65644016-39b1-43b1-bf79-96bc8fe82c15
Make code a bit more robust

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_5_Release@58189 65644016-39b1-43b1-bf79-96bc8fe82c15