[m-dev.] Bug fix in passes_aux.m\

[Andrew Bromage]

G'day.

> If there is a bug that triggers this, even if it won't trigger it until
> you commit the new mode checker, can you keep that around - it's not
> that important now, but any test cases you found while getting the mode
> checker to work are probably worth keeping and committing once you
> committ the new mode checker (or even before)..

The new mode checker found the bug in modes_bug.m, which is already
in tests/valid.

Cheers,
Andrew Bromage