[gfortran] patch PR 20660

[François-Xavier Coudert]

> A unit number must be nonnegative, so it would be better
> to return false for a negative number.

I thought this was already taken care of at this point of the code, but it 
isn't. Updated patch will come.

> In trying this, I found a couple of other oddities that I will send.
> They would be pretty far down the list of things to fix, I would think.

Yes, but as not all of us have the same ability to fix bugs, you will 
probably find people like me that can fix precise, I/O related bugs while 
not being able to mess with the guts of the compiler itself. When you have 
time, I'll be happy to have a look at that.

FX