[krbdev.mit.edu #8726] Directly dereference the pointer certname which may be NULL in parse_fs_options()

[Greg Hudson via RT]

"kinit -X X509_user_identity=, princname" also causes the null deref 
if the KDC offers PKINIT.

There are some ancillary code hygiene issues here:

* On empty input, parse_fs_options() returns 0 without filling in 
idopts->cert_filename and idopts->key_filename.  This is papered over 
by checks in pkinit_get_certs_fs(), which will return 
KRB5KDC_ERR_PREAUTH_FAILED if either field isn't filled in.

* If the second strdup() in parse_fs_options() fails, it returns 
ENOMEM but does leave an allocated value in idopts->cert_filename.  
This could lead to a memory leak if a subsequent pkinit_identities 
value is tried, but under most circumstances I think the allocated 
value will be cleaned up during teardown.