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.
Fixes: d4aacd50e6cf ("Cygwin: console: Add missing input_mutex guard.")
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>