Check for over/underflow while allocating PTS ids

The behavior of signed integer over/underflow is implementation-defined,
but even if the compiler is nice and just wraps around, we could get
ourselves into trouble later on.

Reviewed-on: http://gerrit.openafs.org/10091
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 6c7c185ba3ec6fbdda0651d18868e66edd655809)

Change-Id: I1feb1879cce064760ae061538215b22b6fd55933
Reviewed-on: http://gerrit.openafs.org/10124
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>