[NETFILTER]: {ip,nf}_conntrack_sctp: fix remotely triggerable NULL ptr dereference (CVE-2007-2876)

When creating a new connection by sending an unknown chunk type, we don't
transition to a valid state, causing a NULL pointer dereference in
sctp_packet when accessing sctp_timeouts[SCTP_CONNTRACK_NONE].

Fix by don't creating new conntrack entry if initial state is invalid.

Noticed by Vilmos Nebehaj <vilmos.nebehaj@ramsys.hu>

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>