Fix the length reported as value length.

The length reported as value length for unknown chunks
was actually the chunk length. Therefore it was off by 4.

Change-Id: Ieea79d2c51b4729fc139395174625d1f362d1ee5
Reviewed-on: https://code.wireshark.org/review/5392
Reviewed-by: Michael Tüxen <tuexen@wireshark.org>