[ruby/reline] Initialize @trap_key to suppress warning

https://github.com/ruby/reline/commit/b42c0980d9

lib/reline/line_editor.rb

@@ -576,6 +576,7 @@ class Reline::LineEditor
       @proc_scope = proc_scope
       @width = nil
       @scroll_top = 0
+      @trap_key = nil
     end
 
     def set_cursor_pos(col, row)