[patch, fortran] Fix PR 20935, failed assertion for scalar mask

[FX Coudert]

> The testcase and C language patch  look fine.  I don't
> know m4 well enough to review that part of the patch.
> Perhaps, FX can give it a quick read.

The m4 part looks mostly OK, but I don't have time to test the patch 
completely. So I'd say it's OK after a week (I might have some time 
over the week-end to look into it again).

Thanks Thomas!

FX