This fixes a bug where opening a message sometimes shows it scrolled
down.

The easiest way to trigger this is to open a long message in a mailbox
with no new messages, scroll down, hit <next-new-then-unread> and then
reopen the message.  The pager will "remember" the position and scroll
down.  If you simply exit the message and re-enter it, the position is
not saved.

More annoyingly, once in a very great while I will open a new message
and have the message scrolled down partway.  I believe this is due to
OldHdr happening to point to the same address as the Header of the new
message.

This is somewhat easily triggered on a mailbox with one message, which
you open, scroll down in, hit <next-new-then-unread> to exit, and then
delete/sync.  In another mutt instance, copy the same messsage back to
the mailbox, then open the message in the original mutt instance.  At
least some of the time, the OldHdr pointer matches the new message,
and so it opens scrolled down.

This patch solves the problem by clearing the pager position once all
redirections through the index menu are done.