patch 9.1.0121: Infinite loop or signed overflow with 'smoothscroll'

Problem:  infinite loop in win_update with 'smoothscroll' set when
          window width is equal to textoff, or signed integer overflow
          if smaller.
Solution: don't revalidate wp->w_skipcol in that case, as no buffer text
          is being shown. (Sean Dewar)

Don't instead reset w_skipcol; that would lose the scroll position
within the line, which may be undesirable if the window is made wider
later.

Also include changes from the splitmove PR #14042 that I (in my infinite
Git wisdom) forgot to commit. This includes a change to
Test_window_split_no_room to ensure it doesn't fail for some screen
sizes.

Move Test_smoothscroll_in_zero_width_window to test_scroll_opt.vim, as
that file feels more appropriate.

closes: #14068

Signed-off-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/drawscreen.c

@@ -1537,7 +1537,7 @@ win_update(win_T *wp)
 
     // Make sure skipcol is valid, it depends on various options and the window
     // width.
-    if (wp->w_skipcol > 0)
+    if (wp->w_skipcol > 0 && wp->w_width > win_col_off(wp))
     {
 	int w = 0;
 	int width1 = wp->w_width - win_col_off(wp);