nsswitch: fix crash on null pam change pw response

nsswitch: fix crash on null pam change pw response

The function _pam_winbind_change_pwd crashes due to a null value passed
to the function strcasecmp and denies to login via graphical login
manager. Check for a null value before doing a strcasecmp.

Bug-Ubuntu: https://bugs.launchpad.net/bugs/1003296
Bug: https://bugzilla.samba.org/show_bug.cgi?id=9013
(Desktop Managers (xdm, gdm, lightdm...) crashes with SIGSEGV in
_pam_winbind_change_pwd() when password is expiring)
(cherry picked from commit 47f2211f137688a7c46c4a38571a9f94e59dbf6a)