isl_map_simplify.c: drop_unrelated_constraints: improve error handling
isl_map_simplify.c: drop_unrelated_constraints: improve error handling

The called functions are unlikely to error out because the input
is currently known to be non-NULL, but it does not hurt to check
for errors anyway.  Furthermore, the next commit may very well
cause the inputs to these functions to become NULL.

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

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