[NETFILTER]: SCTP conntrack: fix crash triggered by packet without chunks

When a packet without any chunks is received, the newconntrack variable
in sctp_packet contains an out of bounds value that is used to look up an
pointer from the array of timeouts, which is then dereferenced, resulting
in a crash. Make sure at least a single chunk is present.

Problem noticed by George A. Theall <theall@tenablesecurity.com>

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