DPE: Fix null pointer dereference in debug logging

Changes log_derive_context_output to account for the fact that
new_certificate_actual_size may be NULL if return_certificate is false.

Signed-off-by: Jamie Fox <jamie.fox@arm.com>
Change-Id: Ia1022238b5d17b4559f1c5d4116409e456c69382
3 files changed