libadmin: Don't pass garbage to pts_UserCreate

The libadmin pts_UserCreate function uses the value passed to
it in newUserId to control whether the user is being created
with a user supplied ID or not.

Initialise this value in the caller, so we don't end up creating
users with corrupt ids.

Caught by clang (#985979)

Change-Id: I89492a02db9f981a4d2040ea913a7e75f5f0b272
Reviewed-on: http://gerrit.openafs.org/9401
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>