Null out the socket pointers in X- and port-forwarding channels for

Null out the socket pointers in X- and port-forwarding channels for
which we close the socket without destroying the channel. John
Peterson reports periodic crashes under heavy load which appear to
be fixed by this, though I don't know the exact circumstances
induced by that load.

git-svn-id: http://svn.tartarus.org/sgt/putty@8871 cda61777-01e9-0310-a592-d414129be87e