auth: Avoid double free in key parsing

There was an error path whilst reading an extended key file which could
result in a key being freed using free(key), and then freed again
through the afsconf_typedKey_put() mechanism. Remove this double free.

Caught by clang-analyzer

Change-Id: I40bff56eddf4cb499ae5b7effdaf82f22379109d
Reviewed-on: http://gerrit.openafs.org/9147
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>