[dovecot/core] 7a1c3e: imapc: Fix crash when handling empty mailboxes.

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/dovecot/core
  Commit: 7a1c3ec3711ec8708d0999a4fdda70431dd28341
      https://github.com/dovecot/core/commit/7a1c3ec3711ec8708d0999a4fdda70431dd28341
  Author: Timo Sirainen <timo.sirainen at dovecot.fi>
  Date:   2018-04-11 (Wed, 11 Apr 2018)

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

  Log Message:
  -----------
  imapc: Fix crash when handling empty mailboxes.

Broken by c05b4ac76c4c6012958f1cc37e2fccf7a7d31c33