sktree is indexed on the numeric value of the socket structure's

sktree is indexed on the numeric value of the socket structure's
underlying WinSock SOCKET. Therefore, if we plan to modify the
SOCKET in a socket, we must remove it from the tree before doing so,
and put it back again afterwards. Otherwise it'll violate the tree's
sorting order, and sooner or later someone will try to find it and
get back NULL.

git-svn-id: http://svn.tartarus.org/sgt/putty@7795 cda61777-01e9-0310-a592-d414129be87e