wbb1_thread.polls is decremented twice when deleting posts

    1. Create a new thread without a poll (polls = 0)
    2. Create a reply with a poll (polls = 1)
    3. Trash the reply (polls = 0 because of ThreadEditor::rebuildThreadData not counting trashed posts)
    4. Delete the reply (polls = -1 because of PostAction::delete calling updateCounters for the poll count)

    I suggest that the updateCounters call is replaced by a rebuildThreadData call, possibly the one a few lines above could just be moved down.