Cosmetics.

git-svn-id: https://svn.r-project.org/R/trunk@66518 00db46b3-68df-0310-9c12-caf00c1e9a41
Handle zero length operands.

git-svn-id: https://svn.r-project.org/R/trunk@66519 00db46b3-68df-0310-9c12-caf00c1e9a41