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).