Hi,

I find that bibtex-validate has a minor issue for non-file buffers
with syntax errors:

1. emacs -Q
2. create an empty buffer containing the following text:

@article{1,
  title = {Title},
  author = {authors},
  year = {2019}

3. M-x bibtex-mode, then M-x bibtex-validate
   => Wrong type argument: stringp, nil

This patch fixes the problem. Thanks.