Fix QRESYNC crash after a large number of VANISHED messages.
When handling QRESYNC vanished, imap_expunge_mailbox() was called to empty out the messages. Various counters, including vcount are updated by the mx_update_tables() after the expunge. These counters need to be reset, because mx_update_context() will do so once again at the end of reading new headers. This can cause an out of bounds access in v2r.
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