Clear pager position upon returning to the index menu.
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.
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