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.

Change-Id: I5f963b5623176ac7631693b093d23cf9f0405edc
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>