Fixing a source of evars leak, revealed by contrib QuicksortComplexity
after r16499 which replaced a check for visible undefined evars
(check_evars) by a check for undefined evars in the evar map
(check_evars_are_solved).

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16508 85f007b7-540e-0410-9357-904b9bb8a0f7