patch 9.0.1664: divide by zero when scrolling with 'smoothscroll' set

Problem:    Divide by zero when scrolling with 'smoothscroll' set.
Solution:   Avoid using a negative width. (closes vim#12540, closes vim#12528)

src/move.c

@@ -2591,17 +2591,20 @@ scroll_cursor_bot(int min_scroll, int set_topbot)
 					(curwin, curwin->w_topline, FALSE);
 	    int skip_lines = 0;
 	    int width1 = curwin->w_width - curwin_col_off();
-	    int width2 = width1 + curwin_col_off2();
-	    // 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)
-		skip_lines = 1;
-
-	    top_plines -= skip_lines;
-	    if (top_plines > curwin->w_height)
+	    if (width1 > 0)
 	    {
-		scrolled += (top_plines - curwin->w_height);
+		int width2 = width1 + curwin_col_off2();
+		// 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)
+		    skip_lines = 1;
+
+		top_plines -= skip_lines;
+		if (top_plines > curwin->w_height)
+		{
+		    scrolled += (top_plines - curwin->w_height);
+		}
 	    }
 	}
     }

src/testdir/dumps/Test_smoothscroll_zero_bot.dump

@@ -0,0 +1,19 @@
+| +0#af5f00255#ffffff0||+1#0000000&| +0&&@9
+|@+0#4040ff13&||+1#0000000&| +0&&@9
+|@+0#4040ff13&||+1#0000000&| +0&&@9
+|@+0#4040ff13&||+1#0000000&| +0&&@9
+|@+0#4040ff13&||+1#0000000&| +0&&@9
+|@+0#4040ff13&||+1#0000000&| +0&&@9
+|@+0#4040ff13&||+1#0000000&| +0&&@9
+|@+0#4040ff13&||+1#0000000&| +0&&@9
+|@+0#4040ff13&||+1#0000000&| +0&&@9
+|@+0#4040ff13&||+1#0000000&| +0&&@9
+|@+0#4040ff13&||+1#0000000&| +0&&@9
+|@+0#4040ff13&||+1#0000000&| +0&&@9
+|@+0#4040ff13&||+1#0000000&| +0&&@9
+|@+0#4040ff13&||+1#0000000&| +0&&@9
+|@+0#4040ff13&||+1#0000000&| +0&&@9
+|@+0#4040ff13&||+1#0000000&| +0&&@9
+>@+0#4040ff13&||+1#0000000&| +0&&@9
+|<+3&&| |<+1&&|a|m|e|]| |[|+|]| 
+| +0&&@11

src/testdir/test_scroll_opt.vim

@@ -833,4 +833,28 @@ func Test_smoothscroll_multi_skipcol()
   call StopVimInTerminal(buf)
 endfunc
 
+" this was dividing by zero bug in scroll_cursor_bot
+func Test_smoothscroll_zero_width_scroll_cursor_bot()
+  CheckScreendump
+
+  let lines =<< trim END
+      silent normal yy
+      silent normal 19p
+      winsize 0 19
+      vsplit
+      vertical resize 0
+      set foldcolumn=1
+      set number
+      set smoothscroll
+      silent normal 20G
+  END
+  call writefile(lines, 'XSmoothScrollZeroBot', 'D')
+  let buf = RunVimInTerminal('-u NONE -S XSmoothScrollZeroBot', #{rows: 19, wait_for_ruler: 0})
+  call TermWait(buf, 1000)
+
+  call VerifyScreenDump(buf, 'Test_smoothscroll_zero_bot', {})
+
+  call StopVimInTerminal(buf)
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1664,
 /**/
     1663,
 /**/