[Bug libstdc++/89452] basic_stringbuf::seekoff and basic_stringbuf::seekpos implementations

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89452

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Baykov Nikita from comment #0)
> 4. pptr() is a null pointer. Paragraphs [stringbuf.cons]/3,
> [streambuf.cons]/1, [stringbuf. members]/3 guarantee it.

That's no longer true, see PR 80676. It's still true in the libstdc++
implementation today though.