Fix AudioBuffer::getChannelData implementation

[b-ma]

This function relies on unsafe to be able to mutate the returned channel, which may be a problem in some situations

I think this inhibits some undefined behaviour - theoretically.

You are asking the compiler for a mutable reference of the AudioBuffer, but then this information is 'erased' via the unsafe slice cast. This means if you call the method twice and mutate one of the copies, the compiler could assume the other copy will not be affected.

I don't have a clear solution though (perhaps return the same TypedArray via set_property??) - should we add an issue to address this later?

Originally posted by @orottier in #79 (comment)

[b-ma]

Another weird case to handle:

1. load some AudioBuffer
2. call getChannelData and store to returned Float32Array to use the buffer as an envelop for example
3. AudioBuffer is garbage collected at some point
4. The underlying memory of the Float32Array is somehow reused and the buffer ends up being full of garbage