[dovecot/core] 174242: sdbox: Don't log an error if stub is added twice

[GitHub]

  Branch: refs/heads/master-2.2
  Home:   https://github.com/dovecot/core
  Commit: 174242a162d911a3bc7c0058ae606c169a3d7660
      https://github.com/dovecot/core/commit/174242a162d911a3bc7c0058ae606c169a3d7660
  Author: Timo Sirainen <timo.sirainen at dovecot.fi>
  Date:   2016-11-23 (Wed, 23 Nov 2016)

  Changed paths:
    M src/lib-storage/index/dbox-single/sdbox-file.c
    M src/lib-storage/index/dbox-single/sdbox-file.h
    M src/lib-storage/index/dbox-single/sdbox-save.c

  Log Message:
  -----------
  sdbox: Don't log an error if stub is added twice

There's no locking for them, so it's fine if two processes add the same
mail. The second one could be ignored, but it was a bit easier to just
let it rename over the first one.