patch 9.0.1247: divide by zero with 'smoothscroll' set and a narrow window

Problem:    Divide by zero with 'smoothscroll' set and a narrow window.
Solution:   Bail out when the window is too narrow.
diff --git a/src/move.c b/src/move.c
index d3648df..3c50d25 100644
--- a/src/move.c
+++ b/src/move.c
@@ -1933,6 +1933,9 @@
 	return;
 
     int	    width1 = curwin->w_width - curwin_col_off();
+    if (width1 <= 0)
+	return;  // no text will be displayed
+
     int	    width2 = width1 + curwin_col_off2();
     long    so = get_scrolloff_value();
     int	    scrolloff_cols = so == 0 ? 0 : width1 + (so - 1) * width2;