msm: vidc: Add null pointer checks

msm_vidc_inst pointer instance is used in without any null
pointer check, dereferencing t could device crash.
Fix the issue by adding null pointer checks.

CRs-Fixed: 606486,606469
Change-Id: If21cf7e7391c75f888735d315b16b11f082dd2e7
Signed-off-by: Gayathri Ramaiah <gramai@codeaurora.org>