[dovecot/core] d8ed8d: imapc: Fix potential crash if initial sync fails.

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/dovecot/core
  Commit: d8ed8d5586542aa199bf343cebfe3327c9a472aa
      https://github.com/dovecot/core/commit/d8ed8d5586542aa199bf343cebfe3327c9a472aa
  Author: Timo Sirainen <timo.sirainen at dovecot.fi>
  Date:   2017-06-08 (Thu, 08 Jun 2017)

  Changed paths:
    M src/lib-storage/index/imapc/imapc-sync.c

  Log Message:
  -----------
  imapc: Fix potential crash if initial sync fails.

Happened at least if mailbox had 0 mails and a failing mailbox_sync() was
called twice.

Fixes:
Panic: file imapc-sync.c: line 477 (imapc_sync_index): assertion failed: (mbox->sync_fetch_first_uid == 1)