bug#36263: Fixed in master

[Eli Zaretskii <eliz@gnu.org>]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Mon, 17 Jun 2019 14:33:57 +0200
> 
> This bug is no longer present in master. It would be nice to backport
> a fix to the release branch since it's a crashing bug that's quite
> easy to trigger.

AFAICT, the "fix" on master is quite convoluted, and IMO doesn't
really solve the problem, it just makes it harder to reproduce: when
HOME is not defined, Emacs 27 calls alternative interfaces, but if
those also fail to returns a usable string, it eventually can also
return an empty string, and cause the same assertion violation.

OTOH, I see the same assertion violation in Emacs 25.2, so this is far
from a new problem.

On the third hand, this assertion compiles to nothing in a production
version, so a production version doesn't crash.

Therefore, it sounds like it isn't important enough to fix this on the
release branch, unless I'm missing something.

Thanks.