ipa-kdb: more robust handling of principal addition/editing
    
    The patch addresses the following defect reported by covscan in FreeIPA
    master:
    
    """
    Error: FORWARD_NULL (CWE-476):
    /daemons/ipa-kdb/ipa_kdb_principals.c:1886: assign_zero: Assigning:
    "principal" = "NULL".
    /daemons/ipa-kdb/ipa_kdb_principals.c:1929:
    var_deref_model: Passing null pointer "principal" to "ipadb_entry_to_mods",
    which dereferences it.
    /daemons/ipa-kdb/ipa_kdb_principals.c:1491:9:
    deref_parm_in_call: Function "ipadb_get_ldap_mod_str" dereferences
    "principal".
    /daemons/ipa-kdb/ipa_kdb_principals.c:1174:5:
    deref_parm_in_call: Function "strdup" dereferences "value"
    """
    
    This is a part of series of patches related to
    https://fedorahosted.org/freeipa/ticket/4795
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>