[Bug libstdc++/68197] negative index to ios_base::iword lead to unpredictable result

[mickael.guene at st dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68197

--- Comment #2 from Mickael Guene <mickael.guene at st dot com> ---
 Anyway it's a bad usage since index must come from xalloc.
 I was unable to find what the specifications say in case of using a negative
index (or invalid index), do you have some inputs in this case ?