merge revision(s) r45350: [Backport #9645]

	* 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/branches/ruby_2_0_0@45373 b2dd03c8-39d4-4d8f-98ff-823fe69b080e