vm_insnhelper.c: fix zsuper in prepended

* vm_insnhelper.c (vm_call_method): a method entry refers the based
  class/module, so should search superclass from the origin i-class
  where the entry belongs to, to get rid of infinite loop when zsuper
  in a prepended class/module.  [ruby-core:54105] [Bug #8238]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42724 b2dd03c8-39d4-4d8f-98ff-823fe69b080e