patch 8.2.0634: crash with null partial and blob

Problem:    Crash with null partial and blob.
Solution:   Check for NULL pointer.  Add more tests. (Yegappan Lakshmanan,
            closes #5984)
diff --git a/src/testdir/test_cursor_func.vim b/src/testdir/test_cursor_func.vim
index 7a17958..0ddfcda 100644
--- a/src/testdir/test_cursor_func.vim
+++ b/src/testdir/test_cursor_func.vim
@@ -2,6 +2,7 @@
 
 func Test_wrong_arguments()
   call assert_fails('call cursor(1. 3)', 'E474:')
+  call assert_fails('call cursor(test_null_list())', 'E474:')
 endfunc
 
 func Test_move_cursor()