Avoid assertion violations when scrolling narrowed buffer

Avoid assertion violations when scrolling narrowed buffer

* src/window.c (window_scroll_pixel_based):
* src/xdisp.c (pos_visible_p): Don't allow simulated redisplay to
start outside the accessible portion of the buffer.  This avoids
assertion violations when some Lisp narrows the buffer to less
than the current window, and then attempts to scroll the buffer.