msm: camera: Fix various small issues in vpe driver

Upper and lower bound checks are enforced for num_buff
which is passed from userspace and memset inst_info
to zero to avoid unwanted data passed to userspace.

CRs-fixed: 547927
Change-Id: I0d589ef26e57bb26dc006b5927461edcc95e593e
Signed-off-by: Hariram Purushothaman <hpurus@codeaurora.org>