org-element-export-snippet-parser: Fix when no closing @@

* lisp/org-element.el (org-element-export-snippet-parser): Return nil
when there is no closing @@.
* testing/lisp/test-org-element.el (test-org-element/context): Add
test.

Reported-by: Tom Alexander <tom@fizz.buzz>
Link: https://orgmode.org/list/fb61ea28-f004-4c25-adf7-69fc55683ed4@app.fastmail.com

lisp/org-element.el

@@ -3328,13 +3328,14 @@ Assume point is at the beginning of the snippet."
                  (- contents-end begin))))
 	     (post-blank (skip-chars-forward " \t"))
 	     (end (point)))
-	(org-element-create
-         'export-snippet
-	 (list :back-end backend
-	       :value value
-	       :begin begin
-	       :end end
-	       :post-blank post-blank))))))
+        (when contents-end ; No match when no trailing "@@".
+	  (org-element-create
+           'export-snippet
+	   (list :back-end backend
+	         :value value
+	         :begin begin
+	         :end end
+	         :post-blank post-blank)))))))
 
 (defun org-element-export-snippet-interpreter (export-snippet _)
   "Interpret EXPORT-SNIPPET object as Org syntax."

testing/lisp/test-org-element.el

@@ -4607,7 +4607,13 @@ Text
   (should
    (eq 'link
        (org-test-with-temp-text "* Headline :file<point>:tags: :real:tag:"
-	 (org-element-type (org-element-context))))))
+	 (org-element-type (org-element-context)))))
+  ;; Do not parse partial export snippets.
+  (should-not
+   (eq 'export-snippet
+       (org-test-with-temp-text
+           "<point>@@latex:\n\nparagraph\n\n@@"
+         (org-element-type (org-element-context))))))