msm: vidc: Add null check in msm_venc_stop_streaming

Return from msm_venc_stop_streaming in case of invalid
input arguments. Without this change, we will hit a
NULL pointer exception.

Change-Id: I164d0f27c28d10313475be680ecab56b9f16f429
Signed-off-by: Arun Menon <avmenon@codeaurora.org>