media: staging: atomisp: Check return value from compat_alloc_user_space

If something gets wrong, return, instead of trying to
convert from a NULL pointer.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>