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.

Change-Id: I20ea9c790037a36b8527889b3f7adcfd60e74fd4
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>