tools: kwboot: Fix buffer overflow in kwboot_terminal()

tools: kwboot: Fix buffer overflow in kwboot_terminal()

The `in` variable is set to -1 in kwboot_terminal() if stdin is not a
tty. In this case we should not look whether -1 is set in fd_set, for it
can lead to a buffer overflow, which can be reproduced with
  echo "xyz" | ./tools/kwboot -t /dev/ttyUSB0

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>