Turn IMAP_EXPUNGE_EXPECTED back off when syncing. (closes #3940).

Turn IMAP_EXPUNGE_EXPECTED back off when syncing. (closes #3940).

imap_sync_mailbox() turned on IMAP_EXPUNGE_EXPECTED when issuing a
EXPUNGE command during a sync.  However, it forgot to turn it back off.

That meant that an unexpected EXPUNGE that occurred during a
mx_check_mailbox -> imap_check_mailbox() -> imap_cmd_finish() call was
not setting
  idata->check_status = IMAP_EXPUNGE_PENDING;
and so imap_check_mailbox() was not returning MUTT_REOPENED.

This meant that although the Context had been changed, the index did
not run update_index(), resulting in a possible segfault.

Thanks to Uroš Juvan for reporting the issue, and his invaluable
description of how to reproduce the problem.