RTMP: Return from rtmp_read as soon as some data is available

Earlier, the function only returned when the enough data to fill the
requested buffer was available. This lead to high latency when receiving
low-bandwidth streams.

Originally committed as revision 23642 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/rtmpproto.c

@@ -915,6 +915,7 @@ static int rtmp_read(URLContext *s, uint8_t *buf, int size)
             buf  += data_left;
             size -= data_left;
             rt->flv_off = rt->flv_size;
+            return data_left;
         }
         if ((ret = get_packet(s, 0)) < 0)
            return ret;