dlm: keep listening connection alive with sctp mode

The connection struct with nodeid 0 is the listening socket,
not a connection to another node.  The sctp resend function
was not checking that the nodeid was valid (non-zero), so it
would mistakenly get and resend on the listening connection
when nodeid was zero.

Signed-off-by: Lidong Zhong <lzhong@suse.com>
Signed-off-by: David Teigland <teigland@redhat.com>