Windows: afskfw return error if krb5 not loaded

Windows: afskfw return error if krb5 not loaded

If the Kerberos v5 library cannot be loaded (pkrb5_init_context
equal to NULL) return a reasonable error code instead of
returning success and doing nothing.

Change-Id: I90c106ff6bb4463c3d55d5ac1bbf225464c80c0d
Reviewed-on: http://gerrit.openafs.org/4560
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-on: http://gerrit.openafs.org/4568