sshd: add null check

* For some reason, privsep_pw->pw_passwd is null,
  and trying to run memset() and strlen() on that
  results in a segfault.
* Add a check for null to prevent that.

Change-Id: Ifd8edad787abf4fca481ccf6999bc2106027871a
Merge remote-tracking branch 'cm/cm-10.1' into cm-10.1-linaro