[NETFILTER]: xt_sctp: fix endless loop caused by 0 chunk length

Fix endless loop in the SCTP match similar to those already fixed in
the SCTP conntrack helper (was CVE-2006-1527).

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>