bug#10536: 23.3; Make base64-decode more fault tolerant

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Wed, 18 Apr 2018 00:22:42 +0200
> Cc: 10536@debbugs.gnu.org
> 
> > --- src/fns.c~      2011-04-05 05:46:44.000000000 +0200
> > +++ src/fns.c       2012-01-17 13:59:26.000000000 +0100
> > @@ -3590,7 +3590,8 @@
> >
> >        if (c == '=')
> >     {
> > -     READ_QUADRUPLET_BYTE (-1);
> > +     /* Be tolerant against missing final padding '='.  */
> > +     READ_QUADRUPLET_BYTE (e-to);
> 
> It probably won't harm anything to add this patch...  On the other hand,
> it's not very common to have base64 encoded data that fails in this
> manner.
> 
> What do the rest of you people think?  (I think I'm slightly for
> applying the patch.  "Be liberal in what you receive" and all that.)

Could this "omission" be a sign of malicious stuff in there?  If so,
maybe it's better to introduce a variable that would allow this to be
tolerated, and by default fail with a message telling the user that if
they trust the source of the data, set the variable and retry?