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.
If I interpret the T.128 spec section 9.1 correctly flowMarker is the length that is already read/checked in rdp_read_share_control_header - so the next field would be the padding and then the pduType.