Fix memory leak of statement backend (issue #67)

The leak has been confirmed usign memory debugger. Applied fix suggested
by Evgeny Barabanov to perform allocated statement clean up if exception
is thrown from prepare() method.
Relax test_issue76 assertion (issue #67)

It seems ODBC on Linux is not causing exception as expected - to be
checked
Merge pull request #68 from mloskot/issue-67

Fix memory leak of statement backend (issue #67)