tpm: Check that parse_byte_string() has data to parse

Rather then crashing when there is no data, print an error. The error is
printed by the caller to parse_byte_string().

Acked-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>