[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/protocol.c b/net/sctp/protocol.c
index 0ec234b..13ee7fa 100644
--- a/net/sctp/protocol.c
+++ b/net/sctp/protocol.c
@@ -519,7 +519,8 @@
 /* For v4, the source address is cached in the route entry(dst). So no need
  * to cache it separately and hence this is an empty routine.
  */
-static void sctp_v4_get_saddr(struct sctp_association *asoc,
+static void sctp_v4_get_saddr(struct sctp_sock *sk,
+			      struct sctp_association *asoc,
 			      struct dst_entry *dst,
 			      union sctp_addr *daddr,
 			      union sctp_addr *saddr)