Fix operator= ambiguity on some versions of Clang.

- Earlier versions of Clang (up to at least v3.0) throw an ambiguous
  operator= error in this assignment.  Variations on this error have
  cropped up occassionally in some other projects (e.g. PCL).

Change-Id: I73a632c43528eb69840ce697c55d9afc5f3d8e59