bug#24471: 25.1.50; Error on empty PATH component

[Achim Gratz]

In GNU Emacs 25.1 (x86_64 for Windows)

I've updated from Emacs 24.5 to Emacs 25.1 this morning at work.
Starting the new Emacs produced an error during initialization trying to
process (directory-file-name nil).  The reason it turned out was that
our IT department in their infinite wisdom had ended the system PATH
with a ';'.  Emacs extended that path with ';/usr/bin/' (or something
like that, I'm making this report from home), this created an empty path
element that ended up at directory-file-name.  Emacs should sanitize the
PATH if it can't process nil in downstream functions.



Regards,
Achim.
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