BUG/MEDIUM: net_helper: fix a remaining possibly infinite loop in converters

BUG/MEDIUM: net_helper: fix a remaining possibly infinite loop in converters

The various tcp_option_* converters rely on tcp_fullhdr_find_opt() to
find the option. However, the same bug as fixed in commit dbf471f99a
("BUG/MAJOR: net_helper: ip.fp infinite loop on malformed tcp options")
was also present there, by which an option of length 0 could be looped
over indefinitely. In practice this does not happen since such options
are not valid, but if passed encoded in an HTTP header for example, it
could possibly be passed.

While fixing it, let's check for length >1 in all 3 locations insteead
of only non-zero, since there's no point processing a malformed option
that wouldn't even be properly skipped.

This fix doesn't need to be backported, unless the ip.fp series is.

Thanks to @Vincent55 for reporting this issue.