fixed calculation of length

Author: raydac

src/test/java/com/igormaznitsa/jbbp/it/NetPacketParsingTest.java

@@ -157,7 +157,7 @@ public void testParsingTCPFrameInsideNetworkFrame() throws Exception {
       assertEquals(0, parsedTcpHeader.findFieldForNameAndType("Option", JBBPFieldArrayByte.class).size());
 
       // extract data
-      final int payloadDataLength = ipTotalPacketLength - 20 - (int)netPacketStream.getCounter();
+      final int payloadDataLength = ipTotalPacketLength - (internetHeaderLength*4) - (int)netPacketStream.getCounter();
       final byte [] data = netPacketStream.readByteArray(payloadDataLength);
       assertEquals(119, data.length);