Invalid IP Header Length

<b>Ettercap Version</b>: 1e0ca6b3df6eae81c779a17638a68a2b74bc546b
<b>File</b>: https://www.wireshark.org/download/automated/captures/fuzz-2009-06-26-29078.pcap
<b>Problem</b>: The above file causes a crash. The crash is caused by an infinite ip -> ip call stack eventually causing an overflow. This can occur when the header length in the ip packet is 0. If the header length is 0 then the data does not get incremented at all.
<b>Fix</b>: The minimum valid header length is 20. I added that check and the invalid packet that causes the crash gets ignored.