gsm0808_enc_channel_type: Add spare byte

Related: OS#5911
Change-Id: I6e05e4989924b2258122113eb134e946e9cf0107
diff --git a/src/gsm/gsm0808_utils.c b/src/gsm/gsm0808_utils.c
index 4993539..11e86b8 100644
--- a/src/gsm/gsm0808_utils.c
+++ b/src/gsm/gsm0808_utils.c
@@ -541,6 +541,7 @@
 		break;
 	case GSM0808_CHAN_SIGN:
 		/* Octet 5 is spare */
+		msgb_put_u8(msg, 0);
 		break;
 	default:
 		OSMO_ASSERT(false);