pam: Fix password torching const-ness

pam: Fix password torching const-ness

In some code branches, the PAM code "torches" a password by zeroing
it. However, it does this through a const pointer which we otherwise
know is not actually const. Make sure we get better type checking by
doing this through a non-const pointer.

Reviewed-on: http://gerrit.openafs.org/4554
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 5cd4282758317b24d2f63408ab4c62551bbebc03)

Change-Id: I94b22a31884dc9b184ec094e5cca4b6b0098cb15
Reviewed-on: http://gerrit.openafs.org/6295
Tested-by: Derrick Brashear <shadow@dementix.org>
Reviewed-by: Derrick Brashear <shadow@dementix.org>