Support wrap backwards from end of buffer

[qingshuizheng]

Original func fails to wrap backwards from the end of buffer
while n is negative.

Paving infrastructure for the implementation of following commands:

(defun telega-root-prev-unread ()
    "Move point to the previous chat with unread message."
    (interactive)
    (telega-root-next-unread -1))

(defun telega-root-prev-important()
    "Move point to the previous important chat."
    (interactive)
    (telega-root-next-important -1))

(defun telega-root-prev-mention()
    "Move point to the previous chat with mention."
    (interactive)
    (telega-root-next-mention -1))

(defun telega-root-prev-reaction ()
    "Move point to the previous chat with unread reaction."
    (interactive)
    (telega-root-next-reaction -1))

[zevlg]

Merged in, thanks for the contribution!