isl_vertices.c: triangulate: improve error handling
isl_vertices.c: triangulate: improve error handling

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

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