core: reset scroll after /window {page_down|scroll_down} if last line of buffer is displayed at bottom

A regression was introduced by commit bee56c3b7fa6ef0ce820a8f0a7e5e51b95aa3fab
(/window scroll_beyond_end).

Scenario to reproduce the problem, for example on an IRC channel with a long
history (more than two pages):
  /window page_up
  /window page_down
  <type some text and send to channel>
It will display -MORE(1)- instead of automatically scrolling to bottom of
buffer.