Cygwin: console: Fix exit code for non-cygwin process.

Cygwin: console: Fix exit code for non-cygwin process.

If non-cygwin process is executed in console, the exit code is not
set correctly. This is because the stub process for non-cygwin app
crashes in fhandler_console::set_disable_master_thread() due to NULL
pointer dereference. This bug was introduced by the commit:
3721a756b0d8 ("Cygwin: console: Make the console accessible from
other terminals."), that the pointer cons is accessed before fixing
when it is NULL. This patch fixes the issue.

Fixes: 3721a756b0d8 ("Cygwin: console: Make the console accessible from other terminals.")
Reported-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>