fix stable_norm test: the |small| value was 0 on clang with complex<float>.
fix stable_norm test: the |small| value was 0 on clang with complex<float>.

madlib/eigen_backup,mjbshaw/Eigen,mjbshaw/Eigen,madlib/eigen_backup,rotorliu/eigen,madlib/eigen_backup,mjbshaw/Eigen,rotorliu/eigen,rotorliu/eigen,rotorliu/eigen,madlib/eigen_backup,mjbshaw/Eigen