[dovecot/core] cf3612: storage: do NOT feed p_strconcat() with NULL.

[GitHub]

  Branch: refs/heads/master-2.2
  Home:   https://github.com/dovecot/core
  Commit: cf36129638112c7c9db98f05d4d181c69e40b182
      https://github.com/dovecot/core/commit/cf36129638112c7c9db98f05d4d181c69e40b182
  Author: Baofeng <baofeng.wang at dovecot.com>
  Date:   2016-12-25 (Sun, 25 Dec 2016)

  Changed paths:
    M src/lib-storage/index/dbox-multi/mdbox-storage.c

  Log Message:
  -----------
  storage: do NOT feed p_strconcat() with NULL.

p_strconcat will never take NULL as an valid argument.
So ns->list->set.alt_dir has to be checked NULL.