msm: vidc: add input check for hfi q6 function

The input parameters may be null. This change adds input checks at
function q6_hfi_iface_eventq_read.

CRs-Fixed: 606516
Change-Id: I801a9d3249e9b97fa218fc29bc3a2a0e7a0d4d3f
Signed-off-by: Jorge Solano Altamirano <jsolano@codeaurora.org>