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.

Reviewed-on: http://gerrit.openafs.org/4469
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit fbb4c6115b9af9c52ee06fa9c979a3f4195ad342)

Change-Id: I9a8bccba7b6ecbce393ea149270e5c61ebadd05c
Reviewed-on: http://gerrit.openafs.org/6290
Tested-by: Derrick Brashear <shadow@dementix.org>
Reviewed-by: Derrick Brashear <shadow@dementix.org>