error.c: evaluate RARRAY_AREF once

* error.c (rb_check_backtrace): evaluate RARRAY_AREF only once.
  the first argument of RB_TYPE_P is expanded twice for non-immediate
  types.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41469 b2dd03c8-39d4-4d8f-98ff-823fe69b080e