serial: sandbox: Fix buffer underflow in puts

Fix the buffer underflow that would occur if puts is called with length
of zero.

Fixes: efa51f2bd64 ("serial: sandbox: Implement puts")
Cc: Sean Anderson <sean.anderson@seco.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>
1 file changed