fix session resumption if linked against GnuTLS

gnutls_session_get_data can return GNUTLS_E_SHORT_MEMORY_BUFFER even if
passed a NULL buffer.

Without this patch, lftp cannot list directories or transfer files on
FTP servers that require session resumption on the data connection as
security measure, e.g. if using the latest ProFTPD without
NoSessionReuseRequired set.
Without the patch connections fail with "425 Unable to build data
connection: Operation not permitted" due to the session not being resumed.