deRefStablePtr exception?

[Simon Marlow]

> What exception does deRefStablePtr raise when handed an invalid 
> pointer such as 0?

That's a good point, I don't think it has any error checking at all.  Of
course the only way to get an invalid stable pointer is by calling
freeStablePointer on a valid one, or by using the FFI.

Nevertheless, deRefStablePtr should raise an exception when it finds an
invalid stable pointer.

Cheers,
	Simon