auth: Handle memory errors when parsing key file

Return an IO error if we run out of memory whilst parsing a key file

Caught by clang-analyzer

Change-Id: I783a193fe9315f08d76d1be9b1c77089df25fd64
Reviewed-on: http://gerrit.openafs.org/9146
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>