media: ti-vpe: vpe: Add missing null pointer checks

A few NULL pointer checks were missing.
Add check with appropriate return code.

Signed-off-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>