Networking: In both IPv6 and IPv4 incoming logic: (1) Should check if the...

Networking:  In both IPv6 and IPv4 incoming logic:  (1) Should check if the packet size is large enough before trying to access the packet length in the IP header.  (2) In the comparison between the IP length and the full packet length, need to subtract the size of the link layer header before making the comparison or we will get false positives (i.e., the packet is really too small).