compile.c: disallow next in once

* compile.c (iseq_compile_each0): turned dregx context in "once"
  into "guarded" type from "block" type, to disallow `next`,
  `break`, `redo` as well as outside "once".
  [ruby-core:81805] [Bug #13690]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59202 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_exception.rb

@@ -1080,15 +1080,4 @@ $stderr = $stdout; raise "\x82\xa0"') do |outs, errs, status|
       }
     end;
   end
-
-  def test_break_in_once
-    assert_separately([], "#{<<-"begin;"}\n#{<<~'end;'}")
-    begin;
-      obj = Object.new
-      def obj.try
-        /#{break}/o
-      end
-      assert_raise(LocalJumpError, /proc-closure/) {obj.try}
-    end;
-  end
 end