Windows: netidmgr krb5_cc_get_principal can fail

Windows: netidmgr krb5_cc_get_principal can fail

Do not dereference a NULL pointer if krb5_cc_get_principal fails.

On master this bug is fixed by e55d1774b1b5b27a3617467b5e2a24ee2be3a38c
but that change is after the conversion to the Kerberos Compatibility
SDK and cannot be applied to openafs-stable-1_6_x.

Change-Id: I646011dd3171fd1aa6f40f349329f8261546b809
Reviewed-on: http://gerrit.openafs.org/6019
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>