Heap corruption with ForeignPtr

Looks like #14346 (closed) has been fixed for allocaBytes, but touch# is used in a other places. It could be useful to add some documentation about it to touch#. I noticed the comment on alloca and immediately realized that the same problem might be applicable to withForeignPtr, which it is:

module Main where

import Control.Concurrent
import Control.Monad
import Data.Word
import Foreign.Storable
import Foreign.ForeignPtr
import Numeric

main :: IO ()
main = do
  replicateM_ 49 $ threadDelay 1
  fptr <- mallocForeignPtrBytes 4
  withForeignPtr fptr $ \p ->
    forever $ do
      poke p (0xDEADBEEF :: Word32)
      threadDelay 10
      x <- peek p
      unless (x == 0xDEADBEEF) $ putStrLn (showHex x "")

Compiling it almost always results in a binary that will corrupt memory when executed and eventually results in a segfault:

$ stack --silent --resolver ghc-8.10.1 exec -- ghc -O2 -threaded -fforce-recomp Main.hs && ./Main
[1 of 1] Compiling Main             ( Main.hs, Main.o )
Linking Main ...
^C
$ stack --silent --resolver ghc-8.10.1 exec -- ghc -O2 -threaded -fforce-recomp Main.hs && ./Main
[1 of 1] Compiling Main             ( Main.hs, Main.o )
Linking Main ...
1fee80
4d2463
4d10aa
4d2541
498cd8
4d10aa
416c40
416c40
4d2463
4d2463
1f4370
1f4370
17018
1f4370
416c40
17018
Segmentation fault (core dumped)