Ignore T.128 FlowPDU in Share Control header

According to the Microsoft RDP specification, T.128 flow control PDUs should be ignored when reading Share Control headers. (http://msdn.microsoft.com/en-us/library/cc240576.aspx). This patch checks if we got a flow control PDU (length = 0x8000) and advances the stream to ignore the PDU. The flow control PDU specification is available at http://www.itu.int/rec/T-REC-T.128-199802-S/en.
According to the spec the padding comes before the pduType. However, I have consistently observed it to be the other way around. The only other place that I have seen this code implemented is here:

https://github.com/wallix/redemption/blob/master/core/RDP/share.hpp#L172

They also read the padding before the pduType.