Github user Scotte noticed that in his setup a 'VSS-Monitoring ethernet trailer'
was being added to incoming Ethernet frames that contained SPA packets. This
caused base64 decoding to break because the packet data length was calculated
from what libpcap returned for the frame (so these bytes became included in the
SPA payload itself). This issue was reported as #163 on github.
This commit has fwknopd calculate the SPA payload length using the length field
in the IP header so that any trailing bytes in the Ethernet frame are not
included. This solution also applies to the Ethernet Frame Check Sequence issue
(#126).
e7b48af47c