Attest: Checks challenge in attest_get_option_flags

Fix attest_get_option_flags crashing when it is called with
NULL challenge->ptr (i.e initial_attest_get_token_size
set challenge.ptr=NULL before calling  attest_create_token which
call attest_get_option_flags)

Change-Id: I34fc21ba35802432a041bdb4925c6298106de146
Signed-off-by: Michel Jaouen <michel.jaouen@st.com>