netfilter: nf_ct_ftp: fix out of bounds read in update_nl_seq()

As noticed by Dan Carpenter <error27@gmail.com>, update_nl_seq()
currently contains an out of bounds read of the seq_aft_nl array
when looking for the oldest sequence number position.

Fix it to only compare valid positions.

Cc: stable@kernel.org
Signed-off-by: Patrick McHardy <kaber@trash.net>