Fix problem with constant modulus

[Arnaud Charlet]

> The following two backtraces suggest that gnat1 is blowing up trying to
> set the default_search_dirs.
> 
> #0  0x000647c0 in __gnat_malloc (size=56) at ../../gcc/gcc/ada/s-memory.adb:92

A SEGV in malloc almost always means that free has been called twice
before, or that free has been called with an invalid pointer.

So the error you're looking for occurred before that point.

Arno