bug#34489: 25.2; pdb fails if directory contains '++'

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Sun, 30 Jun 2019 19:24:56 +0200
> Cc: a.soroa@ehu.eus, 34489@debbugs.gnu.org
> 
> > A stab in the dark: gud-pdb-marker-regexp?
> 
> Indeed, that was the culprit.  I've attached a fix.

Hmm... I wonder why the code bothers specifying the allowed characters
explicitly, and in particular why it only allows ASCII characters.
Does the code work if the directory has non-ASCII characters instead
of "++"?

Thanks.