Fix free memory computation
Fix free memory computation

When taking a node from the free list, decrease free memory by its size

jjdekker/libminizinc,nathanielbaxter/libminizinc,nathanielbaxter/libminizinc,nathanielbaxter/libminizinc,tias/libminizinc,jjdekker/libminizinc,tias/libminizinc,jjdekker/libminizinc,nathanielbaxter/libminizinc,jjdekker/libminizinc