Reliably initialise uxnet's socket fd fields to -1.

Reliably initialise uxnet's socket fd fields to -1.

This prevents embarrassing mess-ups involving getting back a Socket
which has mostly been memset to 0 but contains an error message,
sk_close()ing it to free the memory, and finding that standard input
has been closed as a side effect.

git-svn-id: http://svn.tartarus.org/sgt/putty@10073 cda61777-01e9-0310-a592-d414129be87e