asyncio.subprocess: Fix a race condition in communicate()
asyncio.subprocess: Fix a race condition in communicate()

Use self._loop instead of self._transport._loop, because transport._loop is set
to None at process exit.

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