DO NOT MERGE mm-video-v4l2: venc: add checks before accessing heap pointers

Heap pointers do not point to user virtual addresses in case
of secure session.
Set them to NULL and add checks to avoid accesing them

Bug: 28815329
Bug: 28920116

Change-Id: I94fd5808e753b58654d65e175d3857ef46ffba26
3 files changed