volser: Catch EOFs when reading dumps

When reading a standard tag, make sure and catch an EOF error
from iod_getc. Otherwise we'll end up trying to parse 0xFF as a
tag length.

Caught by coverity (#985603)

Reviewed-on: http://gerrit.openafs.org/9275
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit cc703d9562455c74767fcd6d2055339109fa9243)

Change-Id: I29957f7b656bb2cdd19601d90990d7bacd82e98c
Reviewed-on: http://gerrit.openafs.org/9361
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>