ox-latex: Fix numbering of source blocks

* lisp/ox-latex.el (org-latex-src-block): Fix numbering reset on source
  blocks when using the listings package.

Reported-by: P-O Gustafsson <kullager@gmail.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/104650>

lisp/ox-latex.el

@@ -2797,7 +2797,6 @@ contextual information."
 	       `(("captionpos" ,(if caption-above-p "t" "b")))
 	       (cond ((assoc "numbers" lst-opt) nil)
 		     ((not num-start) '(("numbers" "none")))
-		     ((zerop num-start) '(("numbers" "left")))
 		     (t `(("firstnumber" ,(number-to-string (1+ num-start)))
 			  ("numbers" "left"))))))
 	     (let ((local-options (plist-get attributes :options)))