ALSA: firewire-lib: handle the case that empty isochronous packet payload for CIP

Two quadlets are at least included in isochronous packet payload for
Common Isochronous Packet (CIP) format in IEC 61883-1. However, it's
better to equip ALSA IEC 61883-1/6 packet streaming engine for contrary
packet.

This commit handles isochronous cycle to process such packet so that the
cycle is skipped.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210518130048.146596-3-o-takashi@sakamocchi.jpSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/firewire/amdtp-stream.c

@@ -656,11 +656,18 @@ static int parse_ir_ctx_header(struct amdtp_stream *s, unsigned int cycle,
 	}
 
 	if (cip_header_size > 0) {
-		cip_header = ctx_header + 2;
-		err = check_cip_header(s, cip_header, *payload_length,
-				       data_blocks, data_block_counter, syt);
-		if (err < 0)
-			return err;
+		if (*payload_length >= cip_header_size) {
+			cip_header = ctx_header + 2;
+			err = check_cip_header(s, cip_header, *payload_length, data_blocks,
+					       data_block_counter, syt);
+			if (err < 0)
+				return err;
+		} else {
+			// Handle the cycle so that empty packet arrives.
+			cip_header = NULL;
+			*data_blocks = 0;
+			*syt = 0;
+		}
 	} else {
 		cip_header = NULL;
 		err = 0;