Bug 833217: Null out listener if we're no longer listening for DataChannel events r=abr

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extra : transplant_source : H%82K%F1%83%90%228s%5D%16e%F3L%D7%A2%FE%B1%C6%AF
Bug 838169. Fix NrIceCtx cleanup. r=abr

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extra : transplant_source : %E8p%23%5BD%CE%E0%85%8F%B8%40%40x%BA%E9%C5%8B%DE%A1O