tty: wipe buffer.

BugLink: https://bugs.launchpad.net/bugs/1810967

commit c9a8e5fc upstream.

After we are done with the tty buffer, zero it out.
Reported-by: aszlig <aszlig@nix.build>
Tested-by: Milan Broz <gmazyland@gmail.com>
Tested-by: Daniel Zatovic <daniel.zatovic@gmail.com>
Tested-by: aszlig <aszlig@nix.build>
Cc: Willy Tarreau <w@1wt.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/tty/tty_buffer.c

@@ -454,6 +454,8 @@ receive_buf(struct tty_struct *tty, struct tty_buffer *head, int count)
 		if (count && disc->ops->receive_buf)
 			disc->ops->receive_buf(tty, p, f, count);
 	}
+	if (count > 0)
+		memset(p, 0, count);
 	return count;
 }