[SCTP]: Fix potential NULL pointer dereference in sctp_v4_get_saddr

It is possible to get to sctp_v4_get_saddr() without a valid
association.  This happens when processing OOTB packets and
the cached route entry is no longer valid.
However, when responding to OOTB packets we already properly
set the source address based on the information in the OOTB
packet.  So, if we we get to sctp_v4_get_saddr() without an
association we can simply return.

Signed-off-by: Vladislav Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>