ipvs: fix crash with sync protocol v0 and FTP

Fix crash in 3.5+ if FTP is used after switching
sync_version to 0.

Fixes: 749c42b6 ("ipvs: reduce sync rate with time thresholds")
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>