bugfix: do not attempt to parse a corrupt keytab

* This commit avoids the following:

[root@teigiben99 ~]# touch /etc/httpd/conf//HTTP.keytab
[root@teigiben99 ~]# /usr/sbin/cern-get-keytab --isolate --keytab
/etc/httpd/conf//HTTP.keytab --service HTTP --force --verbose
Found KDC: cerndc44.cern.ch
Setting up temporary Kerberos config file
Initializing Kerberos client
Authenticating using keytab file.
using keytab file name: /etc/httpd/conf//HTTP.keytab
resolving credentials cache (MEMORY:cgkccache100611).
resolving keytab file /etc/httpd/conf//HTTP.keytab
scanning keytab file for principal name
cursor is not of type krb5_kt_cursorPtr at /usr/sbin/cern-get-keytab
line 362.