(for 4.9.3) CVE-2018-14468/FRF.16: Add a missing length check.

The specification says in a well-formed Magic Number information element
the data is exactly 4 bytes long. In mfr_print() check this before trying
to read those 4 bytes.

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).