802.11 HCF NAV extends beyond txop limit

[levy]

1. channel is granted to EDCAF, it starts a txop, FS progresses as expected.
2. at the end of txop, last frames duration field extends NAV beyond txop limit.
3. NAV prevents even the same EDCAF to start contention for the channel.

This might be a serious problem if the frame is big enough due to aggregation for example. The result is that the sender may not be able to access the channel for 1ms.