prio-q: initialize cur iterator

For new SubQueues `cur` is not intialized, so front/pop_front will freak
out. I honestly I have no idea how this hasn't been seen, but it was
being triggered frequently on OSX.

Fixes: #6686

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>