Calling `graph_draw` leads to core dump

Hi,

When I try to draw a graph, I get a core dump like this:

python: /usr/include/sparsehash/internal/densehashtable.h:498: \
  void google::dense_hashtable<Value, Key, HashFcn, ExtractKey, SetKey, EqualKey, Alloc>::\
  set_empty_key(google::dense_hashtable<Value, Key, HashFcn, ExtractKey, SetKey, EqualKey, Alloc>::\
  const_reference) [with Value = std::pair<const int, boost::any>; Key = int; \
  HashFcn = std::hash<int>; ExtractKey = google::dense_hash_map<int, boost::any, std::hash<int>, \
  std::equal_to<int>, std::allocator<int> >::SelectKey; \
  SetKey = google::dense_hash_map<int, boost::any, std::hash<int>, std::equal_to<int>, \
  std::allocator<int> >::SetKey; EqualKey = std::equal_to<int>; \
  Alloc = std::allocator<int>; google::dense_hashtable<Value, Key, HashFcn, ExtractKey, SetKey,\
  EqualKey, Alloc>::const_reference = const std::pair<const int, boost::any>&]: \
  Assertion `!settings.use_empty() && "Calling set_empty_key multiple times"' failed.
Aborted (core dumped)

I could not find any core files in my working directory either. I also looked at densehashtable.h, it's these lines:

496  void set_empty_key(const_reference val) {
497    // Once you set the empty key, you can't change it
498    assert(!settings.use_empty() && "Calling set_empty_key multiple times");

I'm on Fedora 23 with sparsehash 2.0.2. I used the attached file (based on the quick start guide) for my test. I have tried with both 2.13 and git master.