Race condition in stopProcess / pCleanup

[sol]

Steps to reproduce:

ghci> import Control.Exception
ghci> import System.Process.Typed
ghci> let config = setStdout createPipe (proc "echo" ["foo"])
ghci> withProcessWait config $ \ p -> Data.ByteString.hGetContents (getStdout p) >> throwIO DivideByZero
*** Exception: waitForProcess: does not exist (No child processes)

We would expect to see DivideByZero but instead get an IOException of type NoSuchThing.

I think this is easy to fix, PR underway.