isl_multi_union_pw_aff_zero: check that input is not a parameter space
isl_multi_union_pw_aff_zero: check that input is not a parameter space

isl_multi_union_pw_aff_zero would only check that the domain
is a set space, but a parameter space is also considered
to be a set space, while it cannot serve as the space of
an isl_multi_union_pw_aff object.

Reviewed-by: Nicolas Vasilache <05af740c82c817c8c65f9e07d08a1b42ef386036@gmail.com>
Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

nicolasvasilache/isl,inducer/isl-mirror,nicolasvasilache/isl,nicolasvasilache/isl,PollyLabs/isl,inducer/isl-mirror,nicolasvasilache/isl,inducer/isl-mirror,nicolasvasilache/isl,PollyLabs/isl,inducer/isl-mirror,PollyLabs/isl,Meinersbur/isl,PollyLabs/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,inducer/isl-mirror,PollyLabs/isl