server: fix register_source when keepalive
server: fix register_source when keepalive

Using the keepalive feature, when a source is lost and when a client disconnects
during the small interval when the source try to reconnect, the disconnected
client is registered again when the source reconnects with sucess, thus there
was an AttributeError since the client sock attribute is set to None.

noirbee/savate,noirbee/savate