Possible race condition in SFBAudioPlayerNode when accessing active decoders

[sbooth]

I believe a similar bug exists in AudioPlayerNode in all methods accessing the current decoder. For example:

SFBAudioEngine/Sources/CSFBAudioEngine/Player/AudioPlayerNode.mm

Lines 457 to 476 in e18876e

	SFBPlaybackTime SFB::AudioPlayerNode::PlaybackTime() const noexcept
	{
	const auto decoderState = GetActiveDecoderStateWithSmallestSequenceNumber();
	if(!decoderState)
	return SFBInvalidPlaybackTime;
	SFBPlaybackTime playbackTime = SFBInvalidPlaybackTime;
	const auto framePosition = decoderState->FramePosition();
	const auto frameLength = decoderState->FrameLength();
	if(const auto sampleRate = decoderState->mSampleRate; sampleRate > 0) {
	if(framePosition != SFBUnknownFramePosition)
	playbackTime.currentTime = framePosition / sampleRate;
	if(frameLength != SFBUnknownFrameLength)
	playbackTime.totalTime = frameLength / sampleRate;
	}
	return playbackTime;
	}

After the decoderState pointer is loaded, it could become invalid (deleted) at any time. Access and deletion of a DecoderState should probably be protected by a lock.

Originally posted by @jlama in #548

[sbooth]

I'm investigating a possible fix in #557