patch 8.1.1636: crash when popup has fitting scrollbar

Problem:    Crash when popup has fitting scrollbar. (Trygve Aaberge)
Solution:   Don't divide by zero if the scrollbar just fits. (closes #4615)

src/popupwin.c

@@ -2463,7 +2463,12 @@ update_popups(void (*win_update)(win_T *wp))
 								   / linecount;
 	    if (sb_thumb_height == 0)
 		sb_thumb_height = 1;
-	    sb_thumb_top = (wp->w_topline - 1 + (linecount / wp->w_height) / 2)
+	    if (linecount <= wp->w_height)
+		// it just fits, avoid divide by zero
+		sb_thumb_top = 0;
+	    else
+		sb_thumb_top = (wp->w_topline - 1
+				+ (linecount / wp->w_height) / 2)
 				* (wp->w_height - sb_thumb_height)
 						  / (linecount - wp->w_height);
 	    if (wp->w_scrollbar_highlight != NULL)

src/testdir/test_popupwin.vim

@@ -1565,6 +1565,18 @@ func Test_popup_scrollbar()
   call delete('XtestPopupScroll')
 endfunc
 
+func Test_popup_fitting_scrollbar()
+  " this was causing a crash, divide by zero
+  let winid = popup_create([
+	\ 'one', 'two', 'longer line that wraps', 'four', 'five'], {
+	\ 'scrollbar': 1,
+	\ 'maxwidth': 10,
+	\ 'maxheight': 5,
+	\ 'firstline': 2})
+  redraw
+  call popup_clear()
+endfunc
+
 func Test_popup_settext()
   if !CanRunVimInTerminal()
     throw 'Skipped: cannot make screendumps'

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1636,
 /**/
     1635,
 /**/