org-lint: Fix wrong warning with inline footnotes

* lisp/org-lint.el (org-lint-undefined-footnote-reference): Do not
  warn when definition is inline.

* testing/lisp/test-org-lint.el (test-org-lint/undefined-footnote-reference):
  Add tests.

Reported-by: Eric S Fraga <esflists@gmail.com>
<http://lists.gnu.org/archive/html/emacs-orgmode/2017-08/msg00577.html>

lisp/org-lint.el

@@ -724,7 +724,7 @@ Use \"export %s\" instead"
     (org-element-map ast 'footnote-reference
       (lambda (f)
 	(let ((label (org-element-property :label f)))
-	  (and label
+	  (and (eq 'standard (org-element-property :type f))
 	       (not (member label definitions))
 	       (list (org-element-property :begin f)
 		     (format "Missing definition for footnote [%s]"

testing/lisp/test-org-lint.el

@@ -304,6 +304,12 @@ This is not a node property
      (org-lint '(undefined-footnote-reference))))
   (should-not
    (org-test-with-temp-text "Text[fn:1]\n[fn:1] Definition"
+     (org-lint '(undefined-footnote-reference))))
+  (should-not
+   (org-test-with-temp-text "Text[fn:1:inline reference]"
+     (org-lint '(undefined-footnote-reference))))
+  (should-not
+   (org-test-with-temp-text "Text[fn::anonymous reference]"
      (org-lint '(undefined-footnote-reference)))))
 
 (ert-deftest test-org-lint/unreferenced-footnote-definition ()