patch 9.0.0667: ml_get error when 'splitkeep' is "screen"

Problem:    ml_get error when 'splitkeep' is "screen". (Marius Gedminas)
Solution:   Check the botline is not too large. (Luuk van Baal,
            closes #11293, closes #11292)
diff --git a/src/testdir/test_window_cmd.vim b/src/testdir/test_window_cmd.vim
index 7718757..61daa8d 100644
--- a/src/testdir/test_window_cmd.vim
+++ b/src/testdir/test_window_cmd.vim
@@ -1807,6 +1807,13 @@
   let top = line('w0')
   help | quit
   call assert_equal(top, line('w0'))
+  " No error when resizing window in autocmd and buffer length changed
+  autocmd FileType qf exe "resize" line('$')
+  cexpr getline(1, '$')
+  copen
+  wincmd p
+  norm dd
+  cexpr getline(1, '$')
 
   %bwipeout!
   set splitbelow&