netfilter: nf_conntrack_proto_sctp: avoid bogus warning

net/netfilter/nf_conntrack_proto_sctp.c: In function 'sctp_packet':
net/netfilter/nf_conntrack_proto_sctp.c:376: warning: array subscript is above array bounds

gcc doesn't realize that do_basic_checks() guarantees that there is
at least one valid chunk and thus new_state is never SCTP_CONNTRACK_MAX
after the loop. Initialize to SCTP_CONNTRACK_NONE to avoid the warning.

Based on patch by Wu Fengguang <wfg@linux.intel.com>

Signed-off-by: Patrick McHardy <kaber@trash.net>