lighttpd: backport fix for plain auth from 1.4.49 release

lighttpd: backport fix for plain auth from 1.4.49 release

Update commit 3d59ce6f502b ("lighttpd: update to 1.4.48") resulted in
plain auth regression: it simply stopped working with:
(mod_auth.c.525) password doesn't match for (...)
appearing on every authentication try.

This regression was fixed in 1.4.49 release. Backport the fix instead of
updating to the 1.4.49 to avoid risking more/other regressions.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 4cc0c8871398d7c2fb879a319c87e320249bfdcc)