Wavetable Synth demo breaks after switching from polyphonic mode

[terryzfeng]

Steps to reproduce:

1. Open Demo: https://googlechromelabs.github.io/web-audio-samples/demos/wavetable-synth/
2. Check box to switch to polyphonic mode
3. Uncheck box to switch back to monophonic

Error message: Uncaught DOMException: Failed to set the 'buffer' property on 'AudioBufferSourceNode': Cannot set buffer to non-null after it has been already been set to a non-null buffer