Fixing #2846: Uncaught exception Reduction.NotArity.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16662 85f007b7-540e-0410-9357-904b9bb8a0f7
Added test for bug #2846.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16663 85f007b7-540e-0410-9357-904b9bb8a0f7