synch'ing message with deleted attachement make it disappear

[Louis-David Mitterrand <vindex+lists-mutt-dev@xxxxxxxxxxx>]

As the subject says, this happens with 1.5.17 and is a very old bug.

To reproduce:

- view imap mailbox
- view message with attachements, delete some
- back to message list
- synch with '$'
- message disappears from list,
- exit with 'x'
- back to message list: message is still there