media: meson: add missing allocation failure check on new_buf

Currently if the allocation of new_buf fails then a null pointer
dereference occurs when assiging new_buf->vb. Avoid this by returning
early on a memory allocation failure as there is not much more can
be done at this point.

Addresses-Coverity: ("Dereference null return")

Fixes: 3e7f51bd ("media: meson: add v4l2 m2m video decoder driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>