org-latex-preview: Add protection when `untrusted-content' is non-nil

* lisp/org.el (org--latex-preview-when-risky): New variable
controlling how to handle LaTeX previews in Org files from untrusted
origin.
(org-latex-preview): Consult `org--latex-preview-when-risky' before
generating previews.

This patch adds a layer of protection when LaTeX preview is requested
for an email attachment, where `untrusted-content' is set to non-nil.

lisp/org.el

@@ -1136,6 +1136,24 @@ the following lines anywhere in the buffer:
   :package-version '(Org . "8.0")
   :type 'boolean)
 
+(defvar untrusted-content) ; defined in files.el
+(defvar org--latex-preview-when-risky nil
+  "If non-nil, enable LaTeX preview in Org buffers from unsafe source.
+
+Some specially designed LaTeX code may generate huge pdf or log files
+that may exhaust disk space.
+
+This variable controls how to handle LaTeX preview when rendering LaTeX
+fragments that originate from incoming email messages.  It has no effect
+when Org mode is unable to determine the origin of the Org buffer.
+
+An Org buffer is considered to be from unsafe source when the
+variable `untrusted-content' has a non-nil value in the buffer.
+
+If this variable is non-nil, LaTeX previews are rendered unconditionally.
+
+This variable may be renamed or changed in the future.")
+
 (defcustom org-insert-mode-line-in-empty-file nil
   "Non-nil means insert the first line setting Org mode in empty files.
 When the function `org-mode' is called interactively in an empty file, this
@@ -15693,6 +15711,7 @@ fragments in the buffer."
   (interactive "P")
   (cond
    ((not (display-graphic-p)) nil)
+   ((and untrusted-content (not org--latex-preview-when-risky)) nil)
    ;; Clear whole buffer.
    ((equal arg '(64))
     (org-clear-latex-preview (point-min) (point-max))