patch 9.1.0406: Divide by zero with getmousepos() and 'smoothscroll'

Problem:  Divide by zero with getmousepos() and 'smoothscroll'.
Solution: Don't compute skip_lines when width1 is zero.
          (zeertzjq)

closes: #14747

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/move.c b/src/move.c
index 6790192..7971848 100644
--- a/src/move.c
+++ b/src/move.c
@@ -2617,12 +2617,14 @@
 			    plines_win
 #endif
 					(curwin, curwin->w_topline, FALSE);
-	    int skip_lines = 0;
 	    int width1 = curwin->w_width - curwin_col_off();
+
 	    if (width1 > 0)
 	    {
 		int width2 = width1 + curwin_col_off2();
-		// similar formula is used in curs_columns()
+		int skip_lines = 0;
+
+		// A similar formula is used in curs_columns().
 		if (curwin->w_skipcol > width1)
 		    skip_lines += (curwin->w_skipcol - width1) / width2 + 1;
 		else if (curwin->w_skipcol > 0)