In ieee802_1x_decapsulate_radius(), eap_server_get_name() may return
NULL, and it could be dereferenced depending on printf implementation.
Change it to return "unknown" instead for the case of no matching EAP
method found. This makes it easier for the callers to simply print this
in logs (which is the only use for this function).
Signed-off-by: Eytan Lifshitz <eytan.lifshitz@intel.com>