[m-rev.] for review: fix bug #407: Exception.getMessage can return null

[Peter Wang]

On Wed, 22 Jun 2016 12:43:12 +1000 (AEST), Julien Fischer <jfischer at opturion.com> wrote:
> 
> For review by Peter.
> 
> -----------------------
> 
> Fix bug #407: Exception.getMessage can return null.
> 
> library/io.m:
>      Add a new wrapper method throw_io_error, that checks whether
>      its argument is null and, if so, converts into into a string
>      before calling ML_throw_io_error.
> 
>      Check whether the Exception.getMessage returns null in a few
>      spots and return a string instead.  (Other uses of getMessage
>      occur in string concatenation expressions where null will
>      automatically be converted into a string.)

That looks fine.

Peter