CVE-2017-13054/LLDP: add a missing length check

In lldp_private_8023_print() the case block for subtype 4 (Maximum Frame
Size TLV, IEEE 802.3bc-2009 Section 79.3.4) did not include the length
check and could over-read the input buffer, put it right.

This fixes a buffer over-read discovered by Bhargava Shastry,
SecT/TU Berlin.

Add a test using the capture file supplied by the reporter(s).
lldp_extract_latlon(const u_char *tptr)
{
    uint64_t latlon;

    latlon = *tptr & 0x3;
    latlon = (latlon << 32) | EXTRACT_32BITS(tptr+1);

    return latlon;
}

lldp_extract_latlon(const u_char *tptr)
{
    uint64_t latlon;

    latlon = *tptr & 0x3;
    latlon = (latlon << 32) | EXTRACT_32BITS(tptr+1);

    return latlon;
}
