patch 9.0.1011: ml_get error when using screenpos()

Problem:    ml_get error when using screenpos().
Solution:   Give an error for the line number. (closes #11661)
diff --git a/src/testdir/test_cursor_func.vim b/src/testdir/test_cursor_func.vim
index a48db15..8bdc956 100644
--- a/src/testdir/test_cursor_func.vim
+++ b/src/testdir/test_cursor_func.vim
@@ -167,6 +167,9 @@
   let pos = screenpos(winid, 1, 66)
   call assert_equal(winrow, pos.row)
   call assert_equal(wincol + 66 + 3, pos.col)
+
+  call assert_fails('echo screenpos(0, 2, 1)', 'E966:')
+
   close
   bwipe!
 endfunc