isl_vertices.c: can_intersect: propagate error from isl_tab_ineq_type
isl_vertices.c: can_intersect: propagate error from isl_tab_ineq_type

Ignoring the error causes a spurious assertion inside isl_tab_add_ineq.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl