bug#7593: 24.0.50; No response from 'occur' (infinite loop when regexp matches empty string?)

[Chong Yidong]

> 1) Start with 'emacs -nw -Q'.  We see buffer *scratch* containing the
> initial greeting ';; This buffer is for notes . . .'.
>
> 2) Enter 'M-x occur<RET>'.  We're now in the minibuffer with prompt
> 'List lines matching regexp:'.
>
> 3) Enter 'a*<RET>'.  Emacs stops responding.

Fixed, thanks.