vim-patch:8.2.3158: strange error message when using islocked() with a number (#28962)

Problem:    Strange error message when using islocked() with a number.
            (Yegappan Lakshmanan)
Solution:   Check that the name is empty.

1840a7b4e3

Use ll_name_len instead.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/eval.c

@@ -1737,6 +1737,7 @@ char *get_lval(char *const name, typval_T *const rettv, lval_T *const lp, const
   }
 
   tv_clear(&var1);
+  lp->ll_name_len = (size_t)(p - lp->ll_name);
   return p;
 }