msm: vidc: Add NULL check for vb buffers

The vb buffer queue may have NULL buffers as they were DQued
by clients. Hence before accessing them check for NULL.

CRs-Fixed: 2136771
Change-Id: I4d99bf24c2d26cf74455692f76b73311cf6f62c6
Signed-off-by: Praneeth Paladugu <ppaladug@codeaurora.org>