Cygwin: console: Fix segfault on shared_console_info access.

Cygwin: console: Fix segfault on shared_console_info access.

- Accessing shared_console_info before initialization causes access
  violation because it is a NULL pointer. The cause of the problem
  reported in https://cygwin.com/ml/cygwin/2020-02/msg00197.html is
  this NULL pointer access in request_xterm_mode_output() when it is
  called from close(). This patch makes sure that shared_console_info
  is not NULL before calling request_xterm_mode_output().