ocfs2: Fix NULL pointer dereferences in o2net

In some situations, ocfs2_set_nn_state might get called with sc = NULL and
valid = 0. If sc = NULL, we can't dereference it to get the o2nm_node
member. Instead, do what o2net_initialize_handshake does and use NULL when
calling o2net_reconnect_delay and o2net_idle_timeout.

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>