preprocessor/7312: bootstrap failure due to invalid use of errno

[Bruno Haible]

Zack Weinberg writes:
> At that point in the code,
> nothing has been done to give errno a meaningful value, so ->err_no
> should just be cleared to 0.

Well, I thought (and still think) that 'errno' here means the error
status from cpp_simplify_path. See the comments at the head of the
function cpp_simplify_path.

I also think that validate_pch() should not ignore this error status.

Bruno