FIX don't hang if one of the process we need to kill has died

lib/XMonad/Internal/Process.hs

@@ -17,6 +17,8 @@ import           Control.Concurrent
 import           Control.Exception
 import           Control.Monad
 
+import           Data.Maybe
+
 import           System.Directory
 import           System.Exit
 import           System.IO
@@ -38,6 +40,8 @@ waitUntilExit pid = do
 
 killHandle :: ProcessHandle -> IO ()
 killHandle ph = do
+  ec <- getProcessExitCode ph
+  unless (isJust ec) $ do
     pid <- getPid ph
     forM_ pid $ signalProcess sigTERM
     -- this may fail if the process exits instantly and the handle