Fix segmentation fault at startup when IPv6 is enabled

Backport commit 5d34493 from upstream.

PR: 243307
Submitted by: Victor Sudakov <vas@sibptus.ru>
Obtained from: Github https://github.com/lavv17/lftp/commit/5d344937d60380341c20409c11f135afb630d7ee
MFH: 2020Q1