[krbdev.mit.edu #8448] Confusing error text for unset default_realm

[Greg Hudson via RT]

The culprit functions here are kdb_get_library_name() and its caller 
krb5_db_setup_lib_handle().  kdb_get_library_name() returns a string 
pointer or NULL, and if it fails (which should usually only happen on a 
failure to get the default realm), krb5_db_setup_lib_handle() returns 
KRB5_KDB_DBTYPE_NOTFOUND with no extended message.