media: mtk-vcodec: potential dereference of null pointer

The return value of devm_kzalloc() needs to be checked.
To avoid use of null pointer in case of thefailure of alloc.

Fixes: 46233e91fa24 ("media: mtk-vcodec: move firmware implementations into their own files")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Tzung-Bi Shih <tzungbi@google.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
