variable.c: avoid memory leak on const redefinition

* variable.c (rb_const_set): delete existing entry on redefinition
  [Bug #9645]
* test/ruby/test_const.rb (test_redefinition): test for leak

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45350 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-03-17

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45351 b2dd03c8-39d4-4d8f-98ff-823fe69b080e