gsm0480: skip length check for 'RELEASE COMPLETE' message

According to GSM 04.80 Section 2.5 'Release complete' Table 2.5,
the 'RELEASE COMPLETE' message payload is optional, so let's drop
the length check in gsm0480_decode_ss_request() for this type.

Change-Id: I63b7f8ce403169a9dbdbdb031db16693de2196d6
diff --git a/src/gsm/gsm0480.c b/src/gsm/gsm0480.c
index 71470e5..d7c2978 100644
--- a/src/gsm/gsm0480.c
+++ b/src/gsm/gsm0480.c
@@ -249,11 +249,18 @@
 int gsm0480_decode_ss_request(const struct gsm48_hdr *hdr, uint16_t len,
 				struct ss_request *req)
 {
+	uint8_t msg_type = hdr->msg_type & 0x3F;
 	int rc = 0;
 
-	if (len < sizeof(*hdr) + 2) {
-		LOGP(0, LOGL_DEBUG, "SS Request is too short.\n");
-		return 0;
+	/**
+	 * GSM 04.80 Section 2.5 'Release complete' Table 2.5
+	 * payload is optional for 'RELEASE COMPLETE' message
+	 */
+	if (msg_type != GSM0480_MTYPE_RELEASE_COMPLETE) {
+		if (len < sizeof(*hdr) + 2) {
+			LOGP(0, LOGL_DEBUG, "SS Request is too short.\n");
+			return 0;
+		}
 	}
 
 	if (gsm48_hdr_pdisc(hdr) == GSM48_PDISC_NC_SS) {