os: Set oc->fd to -1 when connection is closed

This ensures that we don't use the now-closed file descriptor in the
future.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>

os/connection.c

@@ -772,6 +772,7 @@ CloseDownFileDescriptor(OsCommPtr oc)
         _XSERVTransDisconnect(oc->trans_conn);
         _XSERVTransClose(oc->trans_conn);
         oc->trans_conn = NULL;
+        oc->fd = -1;
     }
 }