Fix redisplay with window-start on continuation lines

Fix redisplay with window-start on continuation lines

* src/xdisp.c (pos_visible_p): Return false if the window starts
after CHARPOS.
(compute_window_start_on_continuation_line): Don't return
window-start position that is after point in the buffer, as the
callers don't expect this to happen, and will generally display an
empty window with the cursor in its middle.  (Bug#23871)