[SCTP]: Fix NULL dereference of asoc.

Commit 7cbca67c ("[IPV6]: Support
Source Address Selection API (RFC5014)") introduced NULL dereference
of asoc to sctp_v6_get_saddr in net/sctp/ipv6.c.
Pointed out by Johann Felix Soden <johfel@users.sourceforge.net>.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>