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

[Greg Hudson via RT]

parse_fs_options() checks for an empty input, so may be assuming that 
strtok_r() won't return null the first time.  I found experimentally 
that it does so if the input is "," (the delimiter).  So we can make 
this code perform a null deref with:

  pkinit_identities = FILE:,