sdp: make parse_sdp_sessions() accept optional \r\n at begin

Instead of having to provide a mandatory \r\n at the begin of the SDP
body that is going to be parsed, this is now optional. Thus the SDP can
now start directly with the `v=` line.

This commit also fixes two other issues:
1. the function might underflow if `v=` is found in the first two chars
2. if the SDP has some garbage at the begining, then it is valid, the
   previous function would have validated it

modules/sngtc/sngtc.c

@@ -437,14 +437,14 @@ int store_sngtc_info(struct dlg_cell *dlg, str *body)
 	memset(info, 0, sizeof(*info));
 
 	lock_init(&info->lock);
-	info->caller_sdp.s = shm_malloc(body->len + 2);
+	info->caller_sdp.s = shm_malloc(body->len);
 	if (!info->caller_sdp.s) {
 		LM_ERR("no more shm\n");
 		goto exit;
 	}
 
-	info->caller_sdp.len = body->len + 2; /* SDP parser needs starting CRLF */
-	memcpy(info->caller_sdp.s, body->s - 2, info->caller_sdp.len);
+	info->caller_sdp.len = body->len; /* SDP parser needs starting CRLF */
+	memcpy(info->caller_sdp.s, body->s, info->caller_sdp.len);
 
 	st.s   = (void *)&info;
 	st.len = sizeof(void *);

parser/sdp/sdp.c

@@ -392,9 +392,22 @@ int parse_sdp_session(str *sdp_body, int session_num, str *cnt_disp, sdp_info_t*
 	 * unconditionally.
 	 */
 	bodylimit = body.s + body.len;
-	v1p = find_sdp_line(body.s, bodylimit, 'v');
-	if (v1p == NULL) {
-		LM_ERR("no sessions in SDP\n");
+	if (body.len < 2) {
+		LM_ERR("body too short!\n");
+		return -1;
+	}
+	v1p = body.s;
+	/* skip over the first initial \r\n  (optional) */
+	if (*v1p == '\r' || *v1p == '\n')
+		v1p++;
+	if (*v1p == '\r' || *v1p == '\n')
+		v1p++;
+	if (v1p + 1 >= bodylimit) {
+		LM_ERR("body too short %ld!\n", bodylimit - v1p);
+		return -1;
+	}
+	if (*v1p != 'v' || *(v1p+1) != '=') {
+		LM_ERR("invalid SDP start: [%c%c]\n", *v1p, *(v1p + 1));
 		return -1;
 	}
 	/* get session origin */