org-element: Fix 'search failed ":"' error

* lisp/org-element.el (org-element-context): Fix 'search failed ":"'
  error when point is on a blank line after a keyword.

* testing/lisp/test-org-element.el (test-org-element/context): Add
  tests.

lisp/org-element.el

@@ -4887,7 +4887,7 @@ Providing it allows for quicker computation."
 	 (if (not (member (org-element-property :key element)
 			  org-element-document-properties))
 	     (throw 'objects-forbidden element)
-	   (beginning-of-line)
+	   (goto-char (org-element-property :begin element))
 	   (search-forward ":")
 	   (if (and (>= origin (point)) (< origin (line-end-position)))
 	       (narrow-to-region (point) (line-end-position))

testing/lisp/test-org-element.el

@@ -3087,9 +3087,16 @@ Text
   ;; Find objects in document keywords.
   (should
    (eq 'macro
-       (org-test-with-temp-text "#+DATE: {{{macro}}}"
-	 (progn (search-forward "{")
-		(org-element-type (org-element-context))))))
+       (org-test-with-temp-text "#+DATE: <point>{{{macro}}}"
+	 (org-element-type (org-element-context)))))
+  (should-not
+   (eq 'macro
+       (org-test-with-temp-text "#+DATE: {{{macro}}}\n<point>"
+	 (org-element-type (org-element-context)))))
+  (should-not
+   (eq 'macro
+       (org-test-with-temp-text "#+RANDOM_KEYWORD: <point>{{{macro}}}"
+	 (org-element-type (org-element-context)))))
   ;; Do not find objects in table rules.
   (should
    (eq 'table-row