New fgets handling reports error when stdin gets closed

[Jean-Pierre Flori]

I've made a patch to gracefully handle this case:
https://git.sagemath.org/sage.git/diff/build/pkgs/singular/patches/fgets.patch?id=37e926f8aec3cd8f64d2739cba81aecb163292f7

Feel free to use it.

[han...@mathematik.uni-kl.de]

Thanks, handling of EBADF integrated.