message: Add bounds checking when looking at the stack in TF Registry.
  This was breaking integration with R, fixes
diff: @@ -64,8 +64,12 @@ class Registry(object):
  # stack trace is [this_function, Register(), user_function,...]
  # so the user function is #2.
  stack = tf_stack.extract_stack()
  - user_function = stack[2]
  + stack_index = min(2, len(stack)-1)
  + if stack_index >= 0:
  + user_function = stack[stack_index]
  location_tag = tf_stack.convert_stack([user_function])[0]
  + else:
  + location_tag = "UNKNOWN"
  self._registry[name] = {_TYPE_TAG: candidate, _LOCATION_TAG: location_tag}
  def list(self):
  