vm_backtrace.c: use long

* vm_backtrace.c (rb_debug_inspector_frame_{class,binding,iseq}_get):
  use long as index as well as RARRAY_LEN().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38962 b2dd03c8-39d4-4d8f-98ff-823fe69b080e