empty node assignment error with GCC address sanitizer

[seisowl]

Compile the code "assign_empty.cpp" and run ./test:

// g++ -g -std=c++11 -fsanitize=address -fno-omit-frame-pointer -fstack-protector-all -D_GLIBCXX_DEBUG assign_empty.cpp -lyaml-cpp -o test

#include <yaml-cpp/yaml.h>

int main(int argc, char** argv) {
  YAML::Node a, b;
  a = b;
  return 0;
}