WiFi: Properly parse bitmap sections (CID-1451085)

WiFi: Properly parse bitmap sections (CID-1451085)

Compressed block acks, in the form of 256 bit bitmaps, are parsed
per 64 bit section. Scanning along a section needs to be done by
indexing this section, not the full 256 bits of the complete bitmap.

Change-Id: Id0e6a7299e14be1ad68dd1cf6d736123008854ac
Reviewed-on: https://code.wireshark.org/review/35440
Reviewed-by: Jaap Keuter <jaap.keuter@xs4all.nl>
Petri-Dish: Jaap Keuter <jaap.keuter@xs4all.nl>
Reviewed-by: Richard Sharpe <realrichardsharpe@gmail.com>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>