gpu_array_positive_size_guard: handle NULL argument
gpu_array_positive_size_guard: handle NULL argument

The NULL argument is treated as an error, but no longer results
in a segmentation fault.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

Meinersbur/ppcg,nicolasvasilache/ppcg,nicolasvasilache/ppcg,Meinersbur/ppcg