rtmp: Set correct message stream id when writing as server

rtmp_write is used both for writing outputs as a server. The
rt->listen flag determines which mode we're running in.

Previously, when running as a server, the message stream id would
always be set to 0 for media/metadata messages. This is surprising
given that we have both responded to "createStream()" with a value
of 1 and sent a "Stream Begin 1" to the client. Furthermore, some
client libraries (Red5) seem to trip up on receiving
"@setDataFrame" on stream 0 (and may be correct to assume that
this message would be sent on stream 1).

libavformat/rtmpproto.c

@@ -3049,7 +3049,12 @@ static int rtmp_write(URLContext *s, const uint8_t *buf, int size)
                                              pkttype, ts, pktsize)) < 0)
                 return ret;
 
-            rt->out_pkt.extra = rt->stream_id;
+            // If rt->listen, then we're running as a a server and should
+            // use the ID that we've sent in Stream Begin and in the
+            // _result to createStream.
+            // Otherwise, we're running as a client and should use the ID
+            // that we've received in the createStream from the server.
+            rt->out_pkt.extra = (rt->listen) ? rt->nb_streamid : rt->stream_id;
             rt->flv_data = rt->out_pkt.data;
         }