vm_eval.c: fix misplaced GC guard

* vm_eval.c (vm_call0_body): fix RB_GC_GUARD location
  (check_funcall_exec): ditto

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45282 b2dd03c8-39d4-4d8f-98ff-823fe69b080e