media: venus: use NULL instead of zero for pointers

As reported by sparse:

	drivers/media/platform/qcom/venus/core.c:227:41: warning: Using plain integer as NULL pointer
	drivers/media/platform/qcom/venus/core.c:228:34: warning: Using plain integer as NULL pointer

Two vars are using zero instead of NULL for pointers. Not really
an issue, but using NULL makes it clearer that the init data is
expecting a pointer.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>