Segmentation fault when trying to use mllex and mlyacc

[Stephen Weeks]

> Many thanks to you and Stephen for the fix in mlton-lib.h, which solved
> the problem. I am going to port about 20000 more lines the next two days
> or so and appreciate that you are willing to provide bug fixes so quickly.

You're welcome.

BTW, we're always on the lookout for interesting benchmarks, the
larger the better.  We've already got variants of lex and yacc in our
benchmark suite, but if you have some other large ones we'd love to
hear about them.