ENH: simpler, more consistent checks for tmp validity (#1775)

- Previously considered to be valid() if it was any reference
  (null or non-null) or a non-null pointer.

  This appears to be a holdover from old code (pre-2015) where
  reinterpret_cast<..>(0) was used instead of the NullObject.

  A reference via a null pointer isn't really possible anywhere. Even
  for things like labelList::null(), they now use the NullObject,
  which has a non-zero memory location.

- now simply check for a non-zero memory address. Regardless of
  pointer or referenced object.