Cygwin: pty: Fix transferring type-ahead input between input pipes.

Cygwin: pty: Fix transferring type-ahead input between input pipes.

After the commit 595fcb21ffc0, transferring type-ahead input between
the pipe for cygwin app and the pipe for non-cygwin app will not be
done appropriately when the stdin of the non-cygwin app is not pty.
Due to this issue, sometimes the keyboard input might be lost which
should be sent to cygwin app. This patch fixes the issue.

Fixes: 595fcb21ffc0 ("Cygwin: pty: Fix reading CONIN$ when stdin is not a pty.")
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>