Fix bug in org-inside-LaTeX-fragment-p

* lisp/org.el (org-inside-LaTeX-fragment-p): Also work correctly in
  first paragraph.

lisp/org.el

@@ -19027,9 +19027,7 @@ looks only before point, not after."
   (catch 'exit
     (let ((pos (point))
 	  (dodollar (member "$" (plist-get org-format-latex-options :matchers)))
-	  (lim (progn
-		 (re-search-backward (concat "^\\(" paragraph-start "\\)") nil t)
-		 (point)))
+	  (lim (save-excursion (org-backward-paragraph) (point)))
 	  dd-on str (start 0) m re)
       (goto-char pos)
       (when dodollar