[Reductionops.append_stack_app]: do not allocate a useless array.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16952 85f007b7-540e-0410-9357-904b9bb8a0f7
Prevent [Evarutil.whd_head_evar] from uselessly reallocating arrays.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16953 85f007b7-540e-0410-9357-904b9bb8a0f7