Fixing glitchy kill ring behavior in irb

See ruby/irb#85 for details

Author: kozinski

lib/reline/line_editor.rb

@@ -185,7 +185,7 @@ def reset_variables(prompt = '', encoding:)
     @cleared = false
     @rerender_all = false
     @history_pointer = nil
-    @kill_ring = Reline::KillRing.new
+    @kill_ring ||= Reline::KillRing.new
     @vi_clipboard = ''
     @vi_arg = nil
     @waiting_proc = nil