sdp parser: fix sdp payload attributes cloning

 - ptime attribute was not computed while allocating space
   for the cloned attributes
(cherry picked from commit f1944ead66d854c349fca67da2b13d2be3c7fc33)

parser/sdp/sdp.c

@@ -916,7 +916,8 @@ sdp_payload_attr_t * clone_sdp_payload_attr(sdp_payload_attr_t *attr)
 			attr->rtp_enc.len +
 			attr->rtp_clock.len +
 			attr->rtp_params.len +
-			attr->sendrecv_mode.len;
+			attr->sendrecv_mode.len +
+			attr->ptime.len;
 	clone_attr = (sdp_payload_attr_t*)shm_malloc(len);
 	if (clone_attr == NULL) {
 		LM_ERR("no more shm mem (%d)\n",len);