Remove string length check from sscanf()

[Florian Weimer]

* Dominic Letz:

> I'm reading this source the first time, but I guess to not break
> anything I could introduce a new type of FILE* string-buffer let's say 
> in 'strops_incr.c' that is working incrementally reading one char at
> the time from the underlying string skipping the strlen()...

And maybe special-case the char-by-char check for small buffer sizes
based on the results of a strnlen call.  I agree that this should work.

Even today, applications can use fmemopen and fscanf to avoid the
overhead.

> So that brings me to my question. Is the incremental approach
> something that would get accepted when I prepare a patch? And how /
> where to submit that? (Sorry GitHub generation speaking).

You could send a patch to libc-alpha (git send-email should work).  Note
that the change will exceed the size threshold for FSF copyright
assignment, so you'll have to follow that process as well:

  Contribution Checklist: FSF copyright assignment
  <https://sourceware.org/glibc/wiki/Contribution%20checklist#FSF_copyright_assignment>

Thanks,
Florian