[dovecot/core] 172b6e: sdbox: Don't log an error if stub is added twice

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/dovecot/core
  Commit: 172b6edea36e425b0d0c35a854721ae63f57c289
      https://github.com/dovecot/core/commit/172b6edea36e425b0d0c35a854721ae63f57c289
  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.