[BUG] Zsh crashes when reverse-menu-complete is invoked during complist MENUSELECT without 'menu:' in $compstate[insert]

[Marlon Richert <marlon.richert@xxxxxxxxx>]

Given:
  * complist's MENUSELECT mode is active
  * $compstate[insert] does not start with 'menu:'
When:
  * The user activates reverse-menu-complete.
Then:
  * Zsh crashes.

Expected behavior:
  * Move to the previous completion in the menu.

Steps to reproduce:
% zmodload zsh/complist
% tst() { compadd bar baz; compstate[insert]=1 }
% zle -C tst menu-select tst
% bindkey '^I' tst
% bindkey -M menuselect '^[[Z' reverse-menu-complete
% ba^I^[[Z