mgcp_sdp: restructure mgcp_write_response_sdp() (rtpmap)

The function mgcp_write_response_sdp() generates the rtpmap lines in the
sdp response. Since we will likely support multiple codecs we will need
to generate several rtpmap lines. Therefore it makes sense to split up
that part in a separate function without altering the overall
functionality (yet)

- add static function add_rtpmap() to generate the rtpmap.

Change-Id: I520e2d40fe6294c88bae63dfcbc5238ef98101e2
Related: OS#3442

src/libosmo-mgcp/mgcp_sdp.c

@@ -286,6 +286,24 @@ int mgcp_parse_sdp_data(const struct mgcp_endpoint *endp,
 	return 0;
 }
 
+
+/* Add rtpmap string to the sdp payload, but only when the payload type falls
+ * into the dynamic payload type range */
+static int add_rtpmap(struct msgb *sdp, int payload_type, const char *audio_name)
+{
+	int rc;
+
+	if (payload_type >= 96 && payload_type <= 127) {
+		if (!audio_name)
+			return -EINVAL;
+		rc = msgb_printf(sdp, "a=rtpmap:%d %s\r\n", payload_type, audio_name);
+		if (rc < 0)
+			return -EINVAL;
+	}
+
+	return 0;
+}
+
 /*! Generate SDP response string.
  *  \param[in] endp trunk endpoint.
  *  \param[in] conn associated rtp connection.
@@ -328,12 +346,8 @@ int mgcp_write_response_sdp(const struct mgcp_endpoint *endp,
 		if (rc < 0)
 			goto buffer_too_small;
 
-		/* FIXME: Check if the payload type is from the static range,
-		 * if yes, omitthe a=rtpmap since it is unnecessary */
-		if (audio_name && endp->tcfg->audio_send_name && (payload_type >= 96 && payload_type <= 127)) {
-			rc = msgb_printf(sdp, "a=rtpmap:%d %s\r\n",
-					 payload_type, audio_name);
-
+		if (endp->tcfg->audio_send_name) {
+			rc = add_rtpmap(sdp, payload_type, audio_name);
 			if (rc < 0)
 				goto buffer_too_small;
 		}