Don't sefault when decoding dicts with broken iterators PYTHON-177

Author: RedBeard0531

bson/_cbsonmodule.c

@@ -778,6 +778,12 @@ int write_dict(buffer_t buffer, PyObject* dict, unsigned char check_keys, unsign
  }
  while ((key = PyIter_Next(iter)) != NULL) {
  PyObject* value = PyDict_GetItem(dict, key);
+ if (!value) {
+ PyErr_SetObject(PyExc_KeyError, key);
+ Py_DECREF(key);
+ Py_DECREF(iter);
+ return 0;
+ }
  if (!decode_and_write_pair(buffer, key, value, check_keys, top_level)) {
  Py_DECREF(key);
  Py_DECREF(iter);