ALSA: seq: Prohibit creating ports with special numbers

Some port numbers are special, such as 254 for subscribers and 255 for
broadcast. Return error if application tries to create such a port.

Reviewed-by: Jaroslav Kysela <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

sound/core/seq/seq_clientmgr.c

···
 		port_idx = info->addr.port;
 	else
 		port_idx = -1;
+	if (port_idx >= SNDRV_SEQ_ADDRESS_UNKNOWN)
+		return -EINVAL;
 	err = snd_seq_create_port(client, port_idx, &port);
 	if (err < 0)
 		return err;