segfault bug

[Stephen Weeks]

> If this file is called z99, then you run the program as
> 
>     rico2 @MLton max-heap 770m -- 0 z99 </dev/null
...
> The  last  thing  it  is  doing before the death is growing a Word8array from
> 134,217,728 to 268,435,456 bytes.

That's cutting it really tight.  With max-heap 770m, you have
807,403,520 bytes available.  So, a semispace has 403,701,760 bytes.
If both arrays are alive, that takes 402,653,184 bytes.  Which means
you only have 1,048,576 bytes available for other stuff.  Does this
seem OK?

Perhaps there is some other bug having to do with an almost full
heap.