IOExts.slurpFile fails on zero-length files

[Simon Marlow]

> Unluckily it doesn't look easy to make 'hGetBuf' fail more gracefully
> (you could read 0 bytes after all), because then still the
> 'mallocBytes 0' in 'slurpFile' will probably fail 

How about: hGetBuf with an argument of zero returns trivially with zero,
and slurpFile on a file with size zero returns (nullPtr, 0)?

Cheers,
	Simon