work around ghc segfault

hSetEncoding of a closed handle segfaults.
https://ghc.haskell.org/trac/ghc/ticket/7161

8484c0c197 introduced the crash.
In particular, stdin may get closed (by eg, getContents) and then trying
to set its encoding will crash. We didn't need to adjust stdin's
encoding anyway, but only stderr, to work around
https://github.com/yesodweb/persistent/issues/474

Thanks to Mesar Hameed for assistance related to reproducing this bug.

Database/Handle.hs

@@ -69,7 +69,7 @@ openDb db tablename = do
 	worker <- async (workerThread (T.pack db) tablename jobs)
 	
 	-- work around https://github.com/yesodweb/persistent/issues/474
-	liftIO useFileSystemEncoding
+	liftIO $ fileEncoding stderr
 
 	return $ DbHandle worker jobs
 

Utility/FileSystemEncoding.hs

@@ -10,6 +10,7 @@
 
 module Utility.FileSystemEncoding (
 	useFileSystemEncoding,
+	fileEncoding,
 	withFilePath,
 	md5FilePath,
 	decodeBS,
@@ -63,6 +64,13 @@ useFileSystemEncoding = do
 	hSetEncoding stderr e
 	Encoding.setLocaleEncoding e	
 
+fileEncoding :: Handle -> IO ()
+#ifndef mingw32_HOST_OS
+fileEncoding h = hSetEncoding h =<< Encoding.getFileSystemEncoding
+#else
+fileEncoding h = hSetEncoding h Encoding.utf8
+#endif
+
 {- Marshal a Haskell FilePath into a NUL terminated C string using temporary
  - storage. The FilePath is encoded using the filesystem encoding,
  - reversing the decoding that should have been done when the FilePath