Fixing #2846: Uncaught exception Reduction.NotArity.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16662 85f007b7-540e-0410-9357-904b9bb8a0f7

Backport of 4372b5 by Pierre-Marie Pédrot. Fixed conflicts (git confused by a slightly different context) by Arnaud Spiwack.
Update CHANGES.