Uniform sampling is not deterministic with default seed

[h3ssto]

jhipster-dimacs.zip

Steps to reproduce:

1. ./ddnnife -i jhipster.dimacs --save-ddnnf jhipster.nnf
2. ./ddnnife -i jhipster.nnf -o jhipster.sample urs -n 16384

This will typically yield a sample that contains 12,215 unique configurations, but occasionally (~1 in 50) a sample with 12,209 unique configurations. As the documentation (./ddnnife urs -h) specifies that the seed defaults to 42, this behavior is unexpected.