CHROMIUM: mtk-vcodec: add NULL pointer dereference protection in ipi handler

BUG=chrome-os-partner:57558
TEST=Play video and run VDA unittest.
Signed-off-by: PC Chen <pc.chen@mediatek.com>
Signed-off-by: Wu-Cheng Li <wuchengli@chromium.org>

Change-Id: Iaeb0f8072ab42f02d8e5750d5971c08e3dbac58b
Reviewed-on: https://chromium-review.googlesource.com/388225
Commit-Ready: Wu-cheng Li <wuchengli@chromium.org>
Tested-by: Wu-cheng Li <wuchengli@chromium.org>
Reviewed-by: Pc Chen <pc.chen@mediatek.com>
Reviewed-by: Wu-cheng Li <wuchengli@chromium.org>
(cherry picked from commit 3752592f43add2cdf374978db7b95cce715f730b)
Reviewed-on: https://chromium-review.googlesource.com/393429
2 files changed