[patch, fortran] Fix ICE on invalid with DO steps

[Thomas Koenig]

Hi Steve,

> s/previusly/previously

Fixed, committed. Thanks for the review!

> Do you it gfortran should skip the front-end optimization pass
> if error(s) have already been reported?  On entry ito this pass,
> you could test for the error count and simply return.

I thought about this for doing this here.  It's a front-end pass,
but for error checking and warnings, not an optimization one, so
I thought throwing a few additional errors / warnings would not
hurt.

Regards

	Thomas