Debian DLA-2338-2 : proftpd-dfsg regression update

The update of proftpd-dfsg released as DLA-2338-1 incorrectly destroyed the memory pool in function sftpkexhandle in contrib/modsftp/kex.c which may cause a segmentation fault and thus prevent sftp connections. For Debian 9 stretch, this problem has been fixed in version 1.3.5e+r1.3.5b-4+deb9u2. ...