ox: Fix next/previous element getters

* lisp/ox.el (org-export-get-previous-element,
  org-export-get-next-element): Fix return value for non plain text
  objects in a secondary string.
* testing/lisp/test-ox.el: Add tests.

lisp/ox.el

@@ -5151,14 +5151,11 @@ all of them."
 	 ;; to a secondary string.  We check the latter option
 	 ;; first.
 	 (let ((parent (org-export-get-parent blob)))
-	   (or (and (not (memq (org-element-type blob)
-			       org-element-all-elements))
-		    (let ((sec-value
-			   (org-element-property
-			    (cdr (assq (org-element-type parent)
-				       org-element-secondary-value-alist))
-			    parent)))
-		      (and (memq blob sec-value) sec-value)))
+	   (or (let ((sec-value (org-element-property
+				 (cdr (assq (org-element-type parent)
+					    org-element-secondary-value-alist))
+				 parent)))
+		 (and (memq blob sec-value) sec-value))
 	       (org-element-contents parent))))
 	prev)
     (catch 'exit
@@ -5186,14 +5183,11 @@ them."
 	 ;; An object can belong to the contents of its parent or to
 	 ;; a secondary string.  We check the latter option first.
 	 (let ((parent (org-export-get-parent blob)))
-	   (or (and (not (memq (org-element-type blob)
-			       org-element-all-objects))
-		    (let ((sec-value
-			   (org-element-property
-			    (cdr (assq (org-element-type parent)
-				       org-element-secondary-value-alist))
-			    parent)))
-		      (cdr (memq blob sec-value))))
+	   (or (let ((sec-value (org-element-property
+				 (cdr (assq (org-element-type parent)
+					    org-element-secondary-value-alist))
+				 parent)))
+		 (cdr (memq blob sec-value)))
 	       (cdr (memq blob (org-element-contents parent))))))
 	next)
     (catch 'exit

testing/lisp/test-ox.el

@@ -2723,6 +2723,12 @@ Another text. (ref:text)
 	 (org-element-type
 	  (org-export-get-next-element
 	   (org-element-map tree 'plain-text 'identity info t) info)))))
+  (should
+   (eq 'verbatim
+       (org-test-with-parsed-data "* /italic/ =verb="
+	 (org-element-type
+	  (org-export-get-next-element
+	   (org-element-map tree 'italic 'identity info t) info)))))
   ;; Find next element in document keywords.
   (should
    (eq 'verbatim
@@ -2783,6 +2789,12 @@ Another text. (ref:text)
 	 (org-element-type
 	  (org-export-get-previous-element
 	   (org-element-map tree 'plain-text 'identity info t) info)))))
+  (should
+   (eq 'verbatim
+       (org-test-with-parsed-data "* =verb= /italic/"
+	 (org-element-type
+	  (org-export-get-previous-element
+	   (org-element-map tree 'italic 'identity info t) info)))))
   ;; Find previous element in document keywords.
   (should
    (eq 'verbatim