[dovecot/core] 9bc8d9: auth: do NOT feed p_strconcat() with NULL

[GitHub]

  Branch: refs/heads/master-2.2
  Home:   https://github.com/dovecot/core
  Commit: 9bc8d9b2047f9685a098e26c0f84c9ea75ea1cee
      https://github.com/dovecot/core/commit/9bc8d9b2047f9685a098e26c0f84c9ea75ea1cee
  Author: Baofeng <baofeng.wang at dovecot.com>
  Date:   2016-12-19 (Mon, 19 Dec 2016)

  Changed paths:
    M src/auth/auth.c

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

p_strconcat will never take NULL as an valid argument.
Check possible NULL string before calling it.