socket: null out priv->ssl_ssl on disconnect

This prevents problems when a socket transport is reused for a non-SSL
connection (e.g. glusterd portmapper) after having been used for an SSL
connection (e.g. a brick).  In that case, ssl_ssl will still be non-null from
the first connection, even though the structure it points to has actually been
freed.

I'm not sure why/how we would reuse a socket transport in this way (the case of
SSL after non-SSL has been common for a long time) but recent glusterd changes
seem to have had that effect.

Change-Id: I46f1ff3c409c122478bf720a7f02d92abcc32bed
BUG: 902684
Signed-off-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-on: http://review.gluster.org/4449
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>