sctp: bail from sctp_endpoint_lookup_assoc() if not bound

The sctp_endpoint_lookup_assoc() function uses a port hash
to lookup the association and then checks to see if any of
them are on the current endpoint.  However, if the current
endpoint is not bound, there can't be any associations on
it, thus we can bail early.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>