markdown-live-preview-mode errors for buffers without file

[xiongtx]

Expected behavior

markdown-live-preview-mode should gracefully fail for markdown buffers with no backing file.

Actual behavior

markdown-live-preview-mode throws a wrong-type-argument error.

This is because (markdown-live-preview-get-filename) is nil for such buffers, which causes markdown-live-preview-remove to call file-exists-p on nil.

Steps to reproduce (recipe)

1. Create a markdown-mode buffer without a backing file.
2. Call markdown-live-preview-mode

Backtraces if necessary (M-x toggle-debug-on-error)

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  file-exists-p(nil)
  (if (file-exists-p outfile-name) (progn (delete-file outfile-name)))
  (let ((outfile-name (markdown-live-preview-get-filename))) (if (file-exists-p outfile-name) (progn (delete-file outfile-name))))
  (progn (let ((outfile-name (markdown-live-preview-get-filename))) (if (file-exists-p outfile-name) (progn (delete-file outfile-name)))))
  (if (eq markdown-live-preview-delete-export (quote delete-on-destroy)) (progn (let ((outfile-name (markdown-live-preview-get-filename))) (if (file-exists-p outfile-name) (progn (delete-file outfile-name))))))
  markdown-live-preview-remove()
  (if markdown-live-preview-mode (markdown-display-buffer-other-window (markdown-live-preview-export)) (markdown-live-preview-remove))
  ...  

Describe versions of markdown-mode, Emacs, OS, etc.

• markdown-mode: 20170614.1152
• Emacs: 25.1.1
• OS: Fedora 24

[jrblevin]

Merged. Thanks for the report and patch!