Fix BaseSubprocessTransport._kill_wait()
Fix BaseSubprocessTransport._kill_wait()

Set the _returncode attribute, so close() doesn't try to terminate the process.

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