l2_packet: Fix l2_packet_none (hostapd default)

The sample code here ended up trying to register an eloop socket with fd
== -1. This was not really ever supposed to be used, but it is now also
hitting an assert in eloop. Skip the unnecessary
eloop_register_read_sock() to avoid this.

This was causing issues for hostapd since CONFIG_L2_PACKET is not set by
default. If CONFIG_RSN_PREAUTH=y was not used for CONFIG_L2_PACKET was
not set in .config explicitly, the defaul use of l2_packet_none.c ended
up hitting the newly added assert() in eloop.

Signed-off-by: Jouni Malinen <j@w1.fi>

src/l2_packet/l2_packet_none.c

@@ -84,7 +84,8 @@ struct l2_packet_data * l2_packet_init(
 	 * TODO: open connection for receiving frames
 	 */
 	l2->fd = -1;
-	eloop_register_read_sock(l2->fd, l2_packet_receive, l2, NULL);
+	if (l2->fd >= 0)
+		eloop_register_read_sock(l2->fd, l2_packet_receive, l2, NULL);
 
 	return l2;
 }