thread.c: get rid of invalid ID symbol

* eval.c (rb_frame_last_func): return the most recent frame method
  name.
* thread.c (recursive_list_access): use the last method name,
  instead of the current method name which can be unset in some
  cases, not to use a symbol by the invalid ID.
  [ruby-core:66742] [Bug #10579]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48744 b2dd03c8-39d4-4d8f-98ff-823fe69b080e