If a bsddb.db.DB object is closed before any DBCursor objects
referencing it are closed, python will crash when the cursors are closed
or deleted.

Workaround: never close a database with open cursors.  this is annoying
in unittest code where a tearDown method closes the DB in the exception
handling path on a test failure before the cursors have been closed or
garbage collected.

I've got a test in my sandbox, I'll check it in with the fix as its a
crasher otherwise.