fix decode raw packet flow record header size

I tested the library and some raw packets lose, I search the problem and I see the header size doesnt correct sometimes, I create this fix and solve the problem but I dont know what the exact problem. May be you can solve.
I don't think this is correct. `len(Header)` should equal `HeaderSize`. See http://www.sflow.org/developers/diagrams/sFlowV5FlowData.pdf.