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

[Bean Zhang via RT]

Hi Team,

In parse_fs_options() of krb5-1.16.1/src/plugins/preauth/pkinit/pkinit_identity.c,
After calling strtok_r() to assign pointer certname,
we directly dereference it without checking if it is valid.

We should add pointer validity checking for certname after assigning.

Could someone help to take a look?

Thanks,
Bean