blockmodel: Fix minor issue with zero-weight edges

src/graph/inference/graph_blockmodel.hh

@@ -209,6 +209,9 @@ public:
         entries_op(_m_entries, _emat,
                    [&](auto r, auto s, auto& me, auto& delta)
                    {
+                       if (get<0>(delta) == 0) // can happen with zero-weight
+                           return;             // edges
+
                        beop(false, me);
 
                        if (Add && me == this->_emat.get_null_edge())

src/graph/inference/graph_blockmodel_util.hh

@@ -1408,7 +1408,8 @@ public:
     void insert_edge(const Edge& e, DynamicSampler<Edge>& elist,
                      size_t weight, size_t& pos)
     {
-        if (pos < elist.size() && elist.is_valid(pos) && elist[pos] == e)
+        if ((pos < elist.size() && elist.is_valid(pos) && elist[pos] == e) ||
+            weight == 0)
             return;
         assert(pos >= elist.size() || !elist.is_valid(pos) || elist[pos] != e);
         pos = elist.insert(e, weight);