added assertion for max_depth greather than 0
added assertion for max_depth greather than 0

yingerj/rdflib,marma/rdflib,dbs/rdflib,ssssam/rdflib,RDFLib/rdflib,avorio/rdflib,avorio/rdflib,yingerj/rdflib,dbs/rdflib,ssssam/rdflib,marma/rdflib,marma/rdflib,armandobs14/rdflib,armandobs14/rdflib,dbs/rdflib,RDFLib/rdflib,armandobs14/rdflib,ssssam/rdflib,yingerj/rdflib,RDFLib/rdflib,avorio/rdflib,dbs/rdflib,ssssam/rdflib,avorio/rdflib,armandobs14/rdflib,RDFLib/rdflib,marma/rdflib,yingerj/rdflib