Prevent segmentation fault in notmuch_database_close

Previously opening a notmuch database in read write mode that has been
locked resulted in the notmuch_database_open function executing
notmuch_database_close as a cleanup function. notmuch_database_close
failed to check whether the xapian database has in fact been created.

Add a check whether the xapian database object has actually been
created before trying to call its flush method.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>