[ruby/reline] Suppress warning, "instance variable @ambiguous_width not initialized"

https://github.com/ruby/reline/commit/368f7e2f78

lib/reline.rb

@@ -457,7 +457,7 @@ module Reline
 
     private def may_req_ambiguous_char_width
       @ambiguous_width = 2 if Reline::IOGate == Reline::GeneralIO or STDOUT.is_a?(File)
-      return if @ambiguous_width
+      return if defined? @ambiguous_width
       Reline::IOGate.move_cursor_column(0)
       begin
         output.write "\u{25bd}"