isl_mat_variable_compression: check the number of equality constraints
isl_mat_variable_compression: check the number of equality constraints

isl_mat_variable_compression tacitly assumes that the number of
equality constraints is smaller than or equal to the number of variables.
Make the check explicit.

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

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