msm: vidc: Do not access freed session pointer

During video session shutdown, when venus hfi session is
cleaned up, the session would first be freed and then the
same invalid session handle is used to retrieve the venus
device handle. This patch fixes the device crash issue
arising from access of the invalid session handle.

Change-Id: I320324033e7bfcd5754bbc55ac128b33c8920c28
Signed-off-by: Arun Menon <avmenon@codeaurora.org>