ipv4_payload_length() wrong use of header length
ipv4_payload_length() wrong use of header length

I noticed a discrepancy between packet_size() and get_total_length().
Created a packet of length 84, and packet_size() returned 99.

After some digging, it turned out that ipv4_payload_length() did not
take into account that the header length is expressed in 32 bits words.

Apparently also (requires careful verification), ipv4_packet_test()
had a wrong assertion, too. Took also the opportunity to add a
direct assertion to compare packet_size() and
get_total_length() if I'm right to think they should always be equal.

mrmonday/libpnet,libpnet/libpnet,mrmonday/libpnet,mrmonday/libpnet,mrmonday/libpnet,libpnet/libpnet,libpnet/libpnet,libpnet/libpnet