BUG/MEDIUM: session: properly clean the outgoing connection before freeing.

In session_free(), make sure the outgoing connection is not in the idle list
anymore, and it does no longer have an owner, so that it will properly be
destroyed and nobody will be able to access it.
diff --git a/src/session.c b/src/session.c
index 807af2e..c23d35c 100644
--- a/src/session.c
+++ b/src/session.c
@@ -78,9 +78,12 @@
 	if (conn != NULL && conn->mux)
 		conn->mux->destroy(conn);
 	conn = sess->srv_conn;
-	if (conn != NULL && conn->mux)
+	if (conn != NULL && conn->mux) {
+		LIST_DEL(&conn->list);
+		LIST_INIT(&conn->list);
+		conn->owner = NULL;
 		conn->mux->destroy(conn);
-	else if (conn) {
+	} else if (conn) {
 		/* We have a connection, but not yet an associated mux.
 		 * So destroy it now.
 		 */