bluez5: fix ISO sequence numbering

Pass zero-length packets to the codec. BAP/ISO may use these to indicate
missing data.

Fix A2DP codecs to not parse input with spa_return_val_if_fail, that's
meant for assertions. Just return -EINVAL directly, it's normal that
input data may contain garbage.

spa/plugins/bluez5/a2dp-codec-faststream.c

@@ -301,6 +301,9 @@ static int codec_encode(void *data,
 static SPA_UNUSED int codec_start_decode (void *data,
 		const void *src, size_t src_size, uint16_t *seqnum, uint32_t *timestamp)
 {
+	if (!src_size)
+		return -EINVAL;
+
 	return 0;
 }