NackGenerator's member function still run after NackGenerator being deleted.

[baiyufei]

https://github.com/versatica/mediasoup/blob/master/worker/src/RTC/NackGenerator.cpp#L112
This line may call NackGenerator's destructor.

AddPacketsToNackList(this->lastSeq + 1, seq);
RTC::RtpStreamRecv::OnNackGeneratorKeyFrameRequired()
RTC::RtpStreamRecv::RequestKeyFrame()
std::unique_ptr<RTC::NackGenerator, std::default_deleteRTC::NackGenerator >::reset(RTC::NackGenerator*)
RTC::NackGenerator::~NackGenerator()

https://github.com/versatica/mediasoup/blob/master/worker/src/RTC/NackGenerator.cpp#L116
This line accesses NackGenerator's nacklist(map), however nacklist has been broken.

https://github.com/versatica/mediasoup/blob/master/worker/src/RTC/NackGenerator.cpp#L262
This line can't return and causes 100% CPU usage in mediasoup-worker.