Coverity rxtx/rtp fixes

- destroy struct rtp only if not nullptr (CID 909223) in case of init
failure
- CID 909226 is just a code flow issue that arises when refactoring -
now just single message is dispatched ih this place but there used to
be more types

Author: MartinPulec

src/video_rxtx/rtp.cpp

@@ -362,7 +362,10 @@ initialize_network(const char *addr, int recv_port, int send_port,
 static void
 destroy_rtp_device(struct rtp *network_device)
 {
-        rtp_done(network_device);
+        if (network_device != nullptr) {
+                rtp_done(network_device);
+        }
+        network_device = nullptr;
 }
 
 void rtp_rxtx_set_pbuf_delay(struct rtp_rxtx_common *s, double delay) {

src/video_rxtx/ultragrid_rtp.cpp

@@ -250,14 +250,14 @@ void ultragrid_rtp_video_rxtx::receiver_process_messages()
                 case RECEIVER_MSG_VIDEO_PROP_CHANGED:
                         rtp_rxtx_set_pbuf_delay(m_rtp_common,
                                            1.0 / msg->new_desc.fps);
+                        free_message((struct message *) msg,
+                                     new_response(RESPONSE_OK, nullptr));
                         break;
                 default:
                         assert(0 && "Wrong message passed to "
                                     "ultragrid_rtp_video_rxtx::receiver_"
                                     "process_messages()");
                 }
-
-                free_message((struct message *) msg, r ? r : new_response(RESPONSE_OK, NULL));
         }
 }