Fix _ProactorBasePipeTransport.close()
Fix _ProactorBasePipeTransport.close()

Set the _read_fut attribute to None after cancelling it.

This change should fix a race condition with
_ProactorWritePipeTransport._pipe_closed().

Martiusweb/asyncio,haypo/trollius,vxgmichel/asyncio,1st1/asyncio,1st1/asyncio,ajdavis/asyncio,jashandeep-sohi/asyncio,jashandeep-sohi/asyncio,gvanrossum/asyncio,Martiusweb/asyncio,gvanrossum/asyncio,manipopopo/asyncio,vxgmichel/asyncio,jashandeep-sohi/asyncio,haypo/trollius,manipopopo/asyncio,haypo/trollius,gsb-eng/asyncio,fallen/asyncio,fallen/asyncio,ajdavis/asyncio,gsb-eng/asyncio,Martiusweb/asyncio,ajdavis/asyncio,gvanrossum/asyncio,fallen/asyncio,1st1/asyncio,manipopopo/asyncio,gsb-eng/asyncio,vxgmichel/asyncio