tzlog : Add check to avoid null pointer dereferencing

Do not assign address to structure and then do not traverse private member
 at structure without checking if its null or not. Hence, add the check to
 avoid it.

Change-Id: Ia1d38cb25ea880004954d1bfb7ed439db84abc2f
Signed-off-by: Sonal Aggarwal <quic_sonaagga@quicinc.com>