fix(auth): use NULL instead of 0 for pointer check

This was triggered by sparse tool:
drivers/auth/mbedtls/mbedtls_x509_parser.c:481:42: warning:
 Using plain integer as NULL pointer

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I392316c2a81ef8da7597e35f136e038f152d19d1
1 file changed