[dovecot/core] fcb3cc: lib: Add missing error handling to o_stream_cork()

[GitHub]

  Branch: refs/heads/master-2.2
  Home:   https://github.com/dovecot/core
  Commit: fcb3ccff65b4511cb348cf07046c4b37dd0c56f8
      https://github.com/dovecot/core/commit/fcb3ccff65b4511cb348cf07046c4b37dd0c56f8
  Author: Timo Sirainen <timo.sirainen at dovecot.fi>
  Date:   2017-03-22 (Wed, 22 Mar 2017)

  Changed paths:
    M src/lib/ostream.c

  Log Message:
  -----------
  lib: Add missing error handling to o_stream_cork()

With ostream-file the corking could have called buffer_flush(), which could
have failed and set stream_errno, but nothing would set last_failed_errno.

Fixes:
Panic: file ostream.c: line 59 (o_stream_close_full): assertion failed: (stream->last_failed_errno != 0)