Fix end time shifting past midnight

* lisp/org.el (org-modify-ts-extra): Properly wrap time after 23:59.

This can still generate wrong timestamps, e.g.

  <2015-09-21 Mo 23:00-23:55> => <2015-09-21 Mo 23:05-00:00>

but at least, the error is predictable.

Reported-by: Detlef Steuer <detlef.steuer@gmx.de>
<http://permalink.gmane.org/gmane.emacs.orgmode/101350>

lisp/org.el

@@ -18111,7 +18111,7 @@ When SUPPRESS-TMP-DELAY is non-nil, suppress delays like \"--2d\"."
 	  (setq m (+ m n)))
 	(if (< m 0) (setq m (+ m 60) h (1- h)))
 	(if (> m 59) (setq m (- m 60) h (1+ h)))
-	(setq h (min 24 (max 0 h)))
+	(setq h (mod h 24))
 	(setq ng 1 new (format "-%02d:%02d" h m)))
        ((org-pos-in-match-range pos 6)
 	(setq ng 6 new (car (rassoc (+ n (cdr (assoc (match-string 6 s) idx))) idx))))