pt_util: Catch sscanf failures

If there isn't sufficient data in the input line to satisfy sscanf,
fail with an error, rather than continuing with potentially corrupt
data.

Reviewed-on: http://gerrit.openafs.org/9295
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 62a10e063b4fe6721bd9768611d5f0c13b303189)

Change-Id: I7b9cff1d106538496c6d554291710f73fd6b5370
Reviewed-on: http://gerrit.openafs.org/11022
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>