[DLM] Clear othercon pointers when a connection is closed

This patch clears the othercon pointer and frees the memory when a connnection
is closed. This could cause a small memory leak when nodes leave the cluster.

Signed-Off-By: Patrick Caulfield <pcaulfie@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>