org-html-format-latex: Prevent side effects of copying `buffer-file-name'

* lisp/ox-html.el (org-html-format-latex): Unset `buffer-file-name'
after we are done with transcoding LaTeX.

Reported-by: Rudolf Adamkovič <salutis@me.com>
Link: https://list.orgmode.org/m2k05c579o.fsf@me.com/T/#m949ed054f0f219e2742697c4c4f49fbe07909f5a

lisp/ox-html.el

@@ -2887,13 +2887,15 @@ INFO is a plist containing export properties."
       (insert latex-frag)
       (org-format-latex cache-relpath nil nil cache-dir nil
 		        "Creating LaTeX Image..." nil processing-type)
-      ;; Present save dialogue to be shown for this buffer.  We need
-      ;; to explicitly disable the dialogue because
+      ;; Present save dialogue to be shown for this buffer and prevent
+      ;; Emacs to jump into this buffer when opening
+      ;; `buffer-file-name' file.  We need this because
       ;; `org-export-copy-buffer' copies `buffer-file-name' local
       ;; variable thus making Emacs think that the buffer copy is
       ;; associated with file.  Note that despite `buffer-file-name',
       ;; `org-export-copy-buffer' arranges saving to not perform
       ;; actual writing onto the disk.
+      (setq buffer-file-name nil)
       (restore-buffer-modified-p nil)
       (buffer-string))))