[ruby/reline] Only show dialogs if default external encoding is UTF-8

Fixes a crash in IRB if a dialog is displayed and the default
external encoding is not UTF-8:

/home/jeremy/tmp/reline/lib/reline/line_editor.rb:731:in `write': U+2588 from UTF-8 to US-ASCII (Encoding::UndefinedConversionError)

https://github.com/ruby/reline/commit/f570525ecd

lib/reline/line_editor.rb

@@ -624,6 +624,7 @@ class Reline::LineEditor
 
   DIALOG_DEFAULT_HEIGHT = 20
   private def render_dialog(cursor_column)
+    return unless Encoding.default_external == Encoding::UTF_8
     @dialogs.each do |dialog|
       render_each_dialog(dialog, cursor_column)
     end