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)
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 4e798c6..3983b57 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -1565,6 +1565,18 @@
   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'