Bug #520 Fix heap overflow on zero or 0xFFFF packet length

Add check for packets that report zero packet length. Example
of fix:

    src/tcpprep --auto=bridge --pcap=poc16-get_l2len-heapoverflow --cachefile=/dev/null
    Warning: poc16-get_l2len-heapoverflow was captured using a snaplen of 17 bytes.  This may mean you have truncated packets.
    safe_pcap_next ERROR: Invalid packet length in tcpprep.c:process_raw_packets() line 334: packet length=0 capture length=0
inet_aton(const char *name, struct in_addr *addr)
{
    in_addr_t a = inet_addr(name);
    addr->s_addr = a;
    return a != (in_addr_t)-1;
}

inet_aton(const char *name, struct in_addr *addr)
{
    in_addr_t a = inet_addr(name);
    addr->s_addr = a;
    return a != (in_addr_t)-1;
}
