[SCTP]: Fix NULL dereference of asoc.

Commit 7cbca67c073263c179f605bdbbdc565ab29d801d ("[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>
diff --git a/net/sctp/ipv6.c b/net/sctp/ipv6.c
index e45e44c..e4aac32 100644
--- a/net/sctp/ipv6.c
+++ b/net/sctp/ipv6.c
@@ -299,7 +299,8 @@
 /* Fills in the source address(saddr) based on the destination address(daddr)
  * and asoc's bind address list.
  */
-static void sctp_v6_get_saddr(struct sctp_association *asoc,
+static void sctp_v6_get_saddr(struct sctp_sock *sk,
+			      struct sctp_association *asoc,
 			      struct dst_entry *dst,
 			      union sctp_addr *daddr,
 			      union sctp_addr *saddr)
@@ -318,7 +319,7 @@
 	if (!asoc) {
 		ipv6_dev_get_saddr(dst ? ip6_dst_idev(dst)->dev : NULL,
 				   &daddr->v6.sin6_addr,
-				   inet6_sk(asoc->base.sk)->srcprefs,
+				   inet6_sk(&sk->inet.sk)->srcprefs,
 				   &saddr->v6.sin6_addr);
 		SCTP_DEBUG_PRINTK("saddr from ipv6_get_saddr: " NIP6_FMT "\n",
 				  NIP6(saddr->v6.sin6_addr));