Cygwin: console: Avoid slipping past disable_master_thread check.

If disable_master_thread flag is set between the code checking that
flag not be set and the code acquiring input_mutex, input record is
processed once after setting disable_master_thread flag. This patch
prevents that.

Backported-from: 9bcfd06 (Cygwin: console: Avoid slipping past disable_master_thread check., 2024-02-03)
Fixes: d4aacd5 ("Cygwin: console: Add missing input_mutex guard.")
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/fhandler/console.cc

@@ -4351,8 +4351,6 @@ fhandler_console::set_disable_master_thread (bool x, fhandler_console *cons)
       else
 	return;
     }
-  if (con.disable_master_thread == x)
-    return;
   cons->acquire_input_mutex (mutex_timeout);
   con.disable_master_thread = x;
   cons->release_input_mutex ();