Fix cursor position under scroll-conservatively and overlay strings

Fix cursor position under scroll-conservatively and overlay strings

* src/xdisp.c (try_scrolling): Handle the case where the last
visible screen line of a window displays a before- or after-string
that takes up the whole screen line, and therefore there's no
place to display the cursor, even though the window does seem to
include the position of point.  (Bug#24179)