Change sidebar next/prev-new to look at buffy->new too.

Look at new in addition to msg_unread count, to account for when
$mail_check_stats is unset or when the sidebar only shows the %n
status flag.

sidebar.c

···
       b = Incoming;
     if (!b || (b == HilBuffy))
       break;
-    if (b->msg_unread > 0)
+    if (b->new || b->msg_unread > 0)
       return b;
   } while (b);
 
···
       b = Outgoing;
     if (!b || (b == HilBuffy))
       break;
-    if (b->msg_unread > 0)
+    if (b->new || b->msg_unread > 0)
       return b;
   } while (b);
 
···
     if (!new_only)
       continue;
 
-    if ((b == OpnBuffy) || (b->msg_unread  > 0) ||
+    if ((b == OpnBuffy) || (b->msg_unread  > 0) || b->new ||
         (b == HilBuffy) || (b->msg_flagged > 0))
       continue;