[rtmpdump] rtmpdump broken for Iplayer HD feeds

[emmerson at sdf.lonestar.org]

I am getting 'ERROR: RTMP_ReadPacket, failed to read RTMP packet header'
when using rtmpdump with get_iplayer for HD feeds.
I have tried multiple shows and it doesn't work for any.
Flvstreamer works for the same HD feeds.
I have tried rtmpdump for lower quality versions of the same shows and it
works so it seems to be a problem restricted to HD versions.