pam: Check for null upwd from getpwnam_r

pam: Check for null upwd from getpwnam_r

The POSIX getpwnam_r can yield a NULL struct passwd pointer even when
the returned error code is 0 (in particular, when the requested entry
is not found). Just add a check for a null upwd to make sure we don't
dereference a NULL pointer.

Change-Id: I00e8d6c53e8228f468c984010695b798f5dcf999
Reviewed-on: http://gerrit.openafs.org/4469
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>