patch 9.0.0652: 'smoothscroll' not tested with 'number' and "n" in 'cpo'

Problem:    'smoothscroll' not tested with 'number' and "n" in 'cpo'.
Solution:   Add tests, fix uncovered problem.
diff --git a/src/drawscreen.c b/src/drawscreen.c
index 4706699..fa5d668 100644
--- a/src/drawscreen.c
+++ b/src/drawscreen.c
@@ -1552,6 +1552,26 @@
     init_search_hl(wp, &screen_search_hl);
 #endif
 
+    // Make sure skipcol is valid, it depends on various options and the window
+    // width.
+    if (wp->w_skipcol > 0)
+    {
+	int w = 0;
+	int width1 = wp->w_width - win_col_off(wp);
+	int width2 = width1 + win_col_off2(wp);
+	int add = width1;
+
+	while (w < wp->w_skipcol)
+	{
+	    if (w > 0)
+		add = width2;
+	    w += add;
+	}
+	if (w != wp->w_skipcol)
+	    // always round down, the higher value may not be valid
+	    wp->w_skipcol = w - add;
+    }
+
 #ifdef FEAT_LINEBREAK
     // Force redraw when width of 'number' or 'relativenumber' column
     // changes.