Continue numbering from any previous numbered snippet with +n, even when previous numbered snippet does not immediately precede it.

* org-mode/lisp/org-exp.el (org-export-number-lines):

  Check whether number parameter (this is a numbered block!) is
  non-nil as well as whether cont is nil (this numbered block should
  *not* continue numbering where we left off before!) before resetting
  the count to zero.

  From the docs:

    If you use a `+n' switch, the numbering from the previous
    numbered snippet will be continued in the current one.

  With this change I believe the code complies with the docs.

lisp/org-exp.el

@@ -2731,7 +2731,7 @@ INDENT was the original indentation of the block."
 (defun org-export-number-lines (text &optional skip1 skip2 number cont
 				     replace-labels label-format)
   (setq skip1 (or skip1 0) skip2 (or skip2 0))
-  (if (not cont) (setq org-export-last-code-line-counter-value 0))
+  (if (and number (not cont)) (setq org-export-last-code-line-counter-value 0))
   (with-temp-buffer
     (insert text)
     (goto-char (point-max))