avoid error when calling pause before play (#977)
avoid error when calling pause before play (#977)

This prevents a `Failed to execute 'stop' on 'AudioBufferSourceNode'` when stop() is called before play().
ale-grosselle/wavesurfer.js,agamemnus/wavesurfer.js,agamemnus/wavesurfer.js,ale-grosselle/wavesurfer.js