Fix bug reported by Hannah Schroeter: reading a file lazily using
hGetContents and then closing it using hClose can cause the program to
fall over with a deadlock.

The reason is that when closing the file in lazyRead{Block,Line,Char},
we set the foreign object in the handle to nullFile__, which causes
the finalizer to run (at some point in the future).  The finalizer
takes the MVar in the handle, frees the contents, but never puts the
MVar back.  hClose then tries to take the MVar, and deadlocks.

The solution is not to set the foreign object to nullFile__ in the
first place; I'm not sure why it was done this way, and in fact it
leads to a memory leak.  hClose itself has a similar problem, leading
to a leak of the fileObject.