variable.c, vm_method.c: pin down

* variable.c (rb_alias_variable): ensure pinned down.

* vm_method.c (rb_method_entry_make): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45487 b2dd03c8-39d4-4d8f-98ff-823fe69b080e